1000BASE-LX    Website  Wikipedia       

1000BASE-SX    Website  Wikipedia       

1000BASE-ZX SMF with a distance limitation of 70km.            

1000BASE‑T (aka: IEEE 802.3ab) Twisted-pair cabling (Cat5, Cat5e, Cat6, Cat7) up to 100 meters.   Wikipedia         

100GBASE-ER4 supports 100Gbps on SMF media up to a distance of 40km.            

10BASE2 (aka: cheapernet, thin Ethernet, thinnet, and thinwire) Variant of Ethernet that uses thin coaxial cable terminated with BNC connectors and T connectors. Deprecated by IEEE 802.3 as of 2011.  Wikipedia         

10BASE5 (aka: thick Ethernet or thicknet) first commercially available variant of Ethernet. Used RG-8 coaxial cable and vampire tap network connectors. IEEE 802.3 has deprecated this standard for new installations as of 2003.  Wikipedia         

10GBASE-ER (extended reach) Has a reach of 40 kilometres (25 mi) over engineered links and 30 km over standard links.   Wikipedia         

10GBASE-LR (long range) SMF media type. Maximum distance limitation of 10km.   Wikipedia         

10GBASE-SR (aka: 10GBASE Short Range) Port type for multi-mode fiber and uses 850 nm lasers. Supports 10Gbps up to 300 meters. 10GBASE-SW used for WANs.   Wikipedia         

2001: IPv6 global unicast address.            

2001::/32 Teredo tunneling address.   Wikipedia  IANA       

2002::/16 6to4 address.   Wikipedia         

5 GHz (SHF) 802.11a/n/ac/ad            

5G 5th generation cellular network. Expected to become a standard in the early 2020s  Wikipedia         

60 GHz (EHF) 802.11ad            

8080 An alternate to HTML's port 80.            

::/0 IPv6 local default route            

::/128 (::) Unspecified IPv6 address.            

::1/128 (::1) IPv6 loopback address.            

A DNS host address record.   Wikipedia  RFC 1035       

A5 Used to encrypt GSM cell phone communications.            

AAAA DNS IPv6 host address record.   Wikipedia  Wikipedia  RFC 3596     

access attack An attack that allows unauthorized access to computer resources.   Wikipedia         

Access Control Entry (ACE) (aka: ACL statement) A single line in a ACL. Can have sequence numbers to identify each entry.   Wikipedia         

Access Control List (ACL) Set of rules to block or allow packets. ACLs can also be used for specifying internal hosts for NAT and identifying traffic for QoS. Can have one ACL per interface, per direction, per protocol. Two types, standard ACL and extended ACL. IPv4 ACLs can be named or numbered while IPv6 ACLs can only be named. IPv6 ACLs are equivalent to IPv4 extended ACLs.   Wikipedia         

access control system Verifies a person's identity and that they are an authorized user.   Wikipedia         

Access Layer Where users connect to or "access" the network.   Wikipedia  Netacad  Netacad  Cisco   

Access Point (AP) A connection, typically wireless, into a network.   Wikipedia         

Access Point Name (APN)    Wikipedia         

Access server Device used for remote entry into a network to configure devices.   Wikipedia  YouTube       

acknowledgment number (ACK) 1-bit flag in a TCP header. Acknowledges a valid field.   Wikipedia  Wikipedia       

ACL Statement (aka: Access Control Entry) A single line in an ACL.            

active hub (deprecated) Type of hub that amplifies and cleans up the signal before broadcasting to all other ports. Deprecated by IEEE 802.3 as of 2011.  Wikipedia         

active state Where the router actively searches for a new path to the destination in an EIGRP route. An EIGRP route would go into the active state if the successor is down or if there was no feasible successor.   Wikipedia         

Active Virtual Forwarder (AVF) Each gateway with an assigned virtual MAC address. GLBP group can have up to 4 AVFs. Extra AVFs (more than 4) become SVF.    Wikipedia         

Active Virtual Gateway (AVG) The first router elected as a gateway in a GLBP group. Highest priority is elected. Assigns a virtual MAC address to each member of the GLBP group. Next highest priority of remaining gateways is elected the SVG.   Wikipedia         

active-standby Mode of NIC redundancy that uses only one available NIC at a time.             

ad hoc Wireless LAN topology. When two or more devices are directly connected through a wireless network without extra infrastructure.   Wikipedia  Wikipedia       

Adaptive Security Appliance (ASA)    Wikipedia         

address resolution Used to find the MAC address associated with an IPv6 unicast address. Similar to ARP for IPv4.            

Address Resolution Protocol (ARP) Resolves IPv4 address to MAC address. Maintains a mapping table.   Wikipedia         

adjacency database One of the three OSPF databases. Contains the neighbor table.   Wikipedia         

Administrative Distance (AD) Determines the route to install into the IP routing table. The left side of the brackets represent the AD [1/0]. The trustworthiness of a route source in a routing table. 0 = Connected, 1 = Static, 5 = EIGRP summary, 20 = External BGP (eBGP), 90 = internal EIGRP, 100 = IGRP, 110=OSPF, 115=IS-IS, 120=RIP, 170=External EIGRP, 200=Internal BGP, 255=unknown   Wikipedia         

Advanced Data Communication Control Procedures (ADCCP) Layer 2 protocol.   Wikipedia         

Advanced Research Projects Agency Network (ARPANET) Funded by the U.S. Department of Defense. Helped create the first router. Originated in 1986.  Wikipedia         

African Network Information Centre (AfriNIC)      Website       

AirPrint Allows Apple devices running macOS or iOS to wirelessly print to a compatible printer or print server. First released November 22, 2010.  Wikipedia         

alternate ports (backup) Configured in a blocking state to prevent loops. Neither end of the trunk is a root port.            

American Registry for Internet Numbers (ARIN)  Founded April 18, 1997.  Wikipedia  Website       

anycast (one to-nearest)   Wikipedia         

App Velocity Cisco borderless service. Optimizes the user experience of applications.     Cisco  Cisco     

Apple Filing Protocol (AFP) (formerly: AppleTalk Filing Protocol) Proprietary network protocol and part of the Apple File Service (AFS) that offers file services for macOS and the classic Mac OS. One of several file sharing services including SMB, NFS, FTP, and WebDAV.   Wikipedia         

AppleTalk Proprietary suite of networking protocols developed by Apple for their Macintosh computers.   Wikipedia         

Application Layer (OSI Layer 7) Abstraction layer in the internet protocol suite (TCP/IP) and the OSI model. Allows access to network services such as file transfer, electronic mail, and database access.   Wikipedia         

Application Layer Services Programs that interface with the network and prepare the data for transfer.    Wikipedia         

Area 0 Main area for an OSPF network Also called the backbone. All areas must be connected to area 0.            

Area Border Router (ABR) Has interfaces attached to multiple OSPF areas. Advertises networks learned from the type 1 LSAs to other areas as type 3 LSAs.   Wikipedia         

ARP cache Collection of ARP entries, which map IP addresses to MAC addresses. Created when a hostname is resolved to an IP address and then an IP address is resolved to a MAC address. Either static or dynamic, but mostly dynamic.   Wikipedia  Petri       

arpa Top level domain             

Asia Pacific Network Information Centre (APNIC)    Wikipedia  Website       

asterisk * A route in a routing table that is a candidate for a default route.    Wikipedia         

Asymmetric Digital Subscriber Line (ADSL) Faster download speed than upload speed.   Wikipedia         

Asynchronous Not requiring a clock rate.   Wikipedia         

Asynchronous Transfer Mode (ATM) A packet-switched WAN. Works on public and private networks. Small cells of fixed length are good for voice and video traffic because it maintains good signal.   Wikipedia         

ATM cells Always have a fixed length of 53 bytes. (5 byte header, 48 byte payload.)    Wikipedia         

Automatic Private IP Addressing (APIPA) (aka: auto-IP) Microsoft's name for an IPv4 link-local address. Addresses are assigned in the class B to range. Typically the result of a DHCP error.   TechNet  Wikipedia       

automatic summarization Converts addresses into classful.   Wikipedia         

Autonomous System (AS) Collection of routers under a common administration such as a company or organization.   Wikipedia         

Autonomous System Boundary Router (ASBR) A router located between an OSPF routing domain and a non-OSPF network.   Wikipedia         

AUX port Similar to a console port but configuration occurs through a dial-up connection.   Wikipedia         

backbone cabling Connects the equipment and telecommunication rooms.   Wikipedia         

backbone router Has at least one interface in OSPF area 0.            

Backup Designated Router (BDR)             

backup ports (alternate) Configured in a blocking state to prevent loops.             

Backward Explicit Congestion Notification (BECN) Frame Relay congestion-notification bit.   Wikipedia  Wikipedia       

bandwidth (aka: Throughput) The amount of data that can be transferred in a given period.            

Basic Service Area (BSA) The coverage area where devices in a BSS can communicate.            

Basic Service Set (BSS) One access point connecting all wireless devices.            

Basic Service Set Identification (BSSID) The formal name of a BSS identified with the AP's MAC address.   Wikipedia         

Berkeley Internet Name Domain (BIND) Name daemon. The message format used by BIND is the most widely used DNS format on the internet. Runs on a DNS server. Originally developed in the early 1980s by four UC Berkeley students.  Wikipedia         

best effort (UDP) Packet delivery is not guaranteed. Does not acknowledge if the data is received.            

best path Network path with lowest metric.            

Better Approach To Mobile Adhoc Networking (B.A.T.M.A.N.) Routing protocol.   Wikipedia         

bit interleaving Keeps track of the number and sequence of bits so they can be reassembled on arrival.   Wikipedia         

bits per second (bps) A measurement of the data transfer rate.   Wikipedia         

Bitstream A sequence of bits.   Wikipedia         

Bluetooth (IEEE 802.15) Wireless standard that operates using short-wavelength UHF radio waves in the ISM band from 2.4 to 2.485 GHz to form personal area networks (PANs). Packet-based protocol with a master/slave architecture. One master may communicate with up to seven slaves in a piconet. Devices operate at very low power levels of approximately 1 milliwatt (mW). Invented by Dutch electrical engineer Jaap Haartsen, working for telecom vendor Ericsson in 1994. Named after the Denmark King, Harald Bluetooth because he united Denmark and Norway just as the protocol did for technology.  Wikipedia (Bluetooth)  Wikipedia (Harald Bluetooth)  Website     

Bluetooth 5 Increased signal range to 40 meters (~130 ft.) and offers playback on multiple devices. Announced June 16, 2016. First available in the Samasun Galaxy S8 April 21, 2017. Later availabe on the iPhone 8 September 22, 2017.    Wikipedia  YouTube (MKBHD)     

Bootstrap Protocol (BOOTP) A precursor to the DHCP protocol. BOOTP is a network protocol used to obtain IP address information during bootup   Wikipedia         

Border Gateway Protocol (BGP) Exterior gateway routing protocol, path vector, classes. Designed to share prefix information between different autonomous systems. Version 1 was introduced in June 1989 under RFC 1105.  Wikipedia  RFC 4271  RFC 1105     

Borderless Network The Cisco Borderless Network Architecture is a network solution.   YouTube         

botnet Computers connected to complete certain tasks. Generally looked upon in a negative way as they can be used to send out spam or perform DDoS attacks.   Wikipedia         

bridge Network device that operates at OSI Layer 2. Joins similar topologies and divides network segments into multiple collision domains. Isolates network traffic and prevents unwanted traffic from entering a network segment where there are no recipients. More intelligent that a repeater but a router is needed to stop broadcast or perform path selection.   Wikipedia         

bridge ID (BID) (Bridge priority + MAC address.) Switches with the lowest BID become the root bridge. An exchange of   Wikipedia         

bridge priority Helps in the root bridge election process. Switch with the lowest priority becomes the root bridge. Default on Cisco switches is 32768. 0 is the highest priority.            

Bridge Protocol Data Unit (BPDU) Used by spanning tree to communicate between switches. If a BPDU frame is seen on a PortFast configured interface, it will shut down the interface to prevent a loop.   Wikipedia         

Bring Your Own Device (BYOD)    Wikipedia         

broadband    Wikipedia         

broadband modem             

broadcast address The address that is used to send data to all hosts on a subnet. For a /24 subnet, the broadcast would be            

broadcast domain The area limited broadcasts can be sent. All hosts on a local network. Separated by a router. Can also be separated by VLAN.   Wikipedia         

broadcast multi-access network Multiple router connected over Ethernet.            

broadcast storms When broadcast frames form layer 2 loops and consume all bandwidth. Layer 2 LAN protocols can't eliminate loops on their own. A Layer 3 protocol is needed to fix the problem because they use routing tables.            

broadcast transmission Sending a packet to all nodes in the network. Examples include ARP and DHCP. Usually restricted to the local network. Can be a directed broadcast or limited broadcast.            

Burned In Address (BIA) Also known as a MAC address.   Wikipedia         

C Indicates a directly connected network in a routing table.   Wikipedia         

Cable Modem (CM)             

Cable Modem Termination System (CMTS)             

cable television (CATV) (originally stood for: Community Access Television, Community Antenna Television) System of delivering television programming to paying subscribers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fiber-optic cables.   Wikipedia         

Carrier Sense Multiple Access (CSMA) Protocols based on CSMA are non-deterministic. (Media independent.)            

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)    Wikipedia         

Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) Can increase latency in a WLAN. Adds a random delay before transmitting data in an attempt to avoid collisions. Sends a message across the media before it sends a frame.   Wikipedia         

Catalyst Brand of network switch from Cisco.   Website  Wikipedia       

Category X cable (Cat X) Type of unshielded twisted pair (UTP) cable. 

Cat 1 Used for telephone wiring. 1 MHz maximum transmission, which is not suitable for data.
Cat 2 4 MHz maximum transmission. (4Mbits)
Cat 3 Used for telephone wiring. Was also used for computer networks during the 1990s.
Cat 4 20 MHz maximum transmission. (16Mbps)
Cat 5/5e 100 MHz maximum transmission. Up to 1000BASE-T (Gigabit Ethernet).
Cat 6 250 MHz maximum transmission. Up to 10GBASE-T (10-Gigabit Ethernet).
Cat 7
Cat 8
  Wikipedia (Cat 3)  Wikipedia (Cat 5)  Wikipedia (Cat 6)  Wikipedia (Cat 7)   

Central Office (CO) Local service provider building that connects the CPE to the provider network.   Wikipedia         

Channel Service Unit/Data Service Unit (CSU/DSU) Layer 1 device. Digital leased lines require both a CSU and a DSU, even if in separate devices. CSU terminates digital signal. DSU converts line frames into frames readable by the LAN.   Wikipedia         

circuit-switched network Uses a dedicated path during transmission. PSTN and ISDN are two examples of a circuit-switched WAN.   Wikipedia         

Cisco Adaptive Security Appliance (ASA) Firewall from Cisco. Two firewall modes of operation: routed mode and transparent mode. Basic configs:

 Replaced the Cisco PIX.  Wikipedia  Netacad       

Cisco Catalyst 2960-X Series of network switch from Cisco. Announced June 4, 2013. Released July 2013.  Website  Wikipedia  Cisco Blog     

Cisco Catalyst 2960X-24PD-L 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Catalyst 2960X-24PS-L 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Catalyst 2960X-24PSQ-L 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Catalyst 2960X-24TD-L 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Catalyst 2960X-24TS-L 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Catalyst 2960X-24TS-LL 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Catalyst 2960X-48FPD-L 24-port network switch from Cisco. Released May 24, 2013.  Website         

Cisco Certified Entry Networking Technician (CCENT)    Website  Wikipedia       

Cisco Certified Internetwork Expert (CCIE)    Wikipedia         

Cisco Certified Network Associate (CCNA)    Wikipedia         

Cisco Certified Network Professional (CCNP)    Wikipedia         

Cisco Common Classification Policy Language (C3PL) Configures ZBF policies.   Cisco         

Cisco Connection Online (CCO) Account used on   Cisco         

Cisco Express Forwarding (CEF) Default forwarding mechanism used on IOS 12.0 or later.   Wikipedia         

Cisco Identity Service Engine             

Cisco Unified Computer System (Cisco USC) Line of scalable servers form Cisco. Announced March 2009. First installed September 1, 2009.  Wikipedia  Cisco Press Release       

Class A address block -   Wikipedia  Wikipedia       

Class B address block -   Wikipedia  Wikipedia       

Class C address block -   Wikipedia  Wikipedia       

Class D address block Multicast addresses.            

Class E address block Experimental addresss.            

classful Only Class A, B, or C address. (,,   Wikipedia         

classless Any class of address. (/8, /25, /30...)            

Classless Inter-Domain Routing (CIDR)    RFC 1519         

clock rate The rate data is transmitted. Configured on DCE interfaces.   Wikipedia         

clock signal    Wikipedia         

clock skew An issue along parallel communication where information does not arrive at the same time.   Wikipedia         

coaxial    Wikipedia         

Code-Division Multiple Access (CDMA)    Wikipedia         

Collaboration Meeting Room (CMR) Collaboration tool combining video, voice, and content sharing.   Cisco  YouTube       

collapsed core model Core and distribution layers collapsed into a single device.   YouTube         

collision When two or more demands are made simultaneously on equipment that can handle only one at any given instant. (i.e. collision domain)   Wikipedia         

collision domain Physical network segment where data packets may collide with one another while being sent. CSMA/CD and CSMA/CA can be used to detect or avoid collisions on Ethernet or wireless LANs respectively.   Wikipedia         

Common Address Redundancy Protocol (CARP) Open standard for first-hop redundancy. Provides functionality similar to VRRP and to Cisco's HSRP.   Wikipedia         

CompTIA Network+  Released February 28, 2015.  Website         

congestion control 3 frame-relay congestion-notification bit. (FECN, BECN, )   Wikipedia         

connection oriented Must establish a connection before sending data.   Wikipedia         

console cable Connects through the console port to a host for configuring the device over CLI.            

Content Delivery Network (CDN) Globally distributed network of proxy servers deployed in multiple data centers.   Wikipedia         

Content-Addressable Memory (CAM) table MAC table   Wikipedia         

Control And Provisioning of Wireless Access Points (CAPWAP)    Wikipedia         

Controlled media access (controlled access) Devices take turns accessing the media with a token. Can be inefficient with too many devices.            

converged network One that transports multiple types of traffic such as voice, video, and data. Most modern netowrks are converged.            

convergence The process of failing over from one route to a backup route.            

Core Layer The backbone layer between networks. Everything must be fast in this layer.   Wikipedia  Netacad  Netacad     

crimper Tool used to make twisted pair network cables.            

Crossover Cable (T568A to T568B) An uncommon networking cable. Directly connect like devices together.    Wikipedia         

crosstalk Interference between twisted-pair copper wires caused by the magnetic field around the adjacent pair of wire.   Wikipedia         

Customer Premises Equipment (CPE) Devices and wiring located on the enterprise edge, connected to a carrier link.   Wikipedia         

Data Communications Equipment (DCE) Female end of a serial connection. (used for clock rate. modem) Devices that put data on the local loop.   Wikipedia         

Data Encapsulation    Wikipedia  YouTube       

Data Stream Interface (DSI) Session layer used to carry Apple Filing Protocol traffic over TCP.   Wikipedia         

Data Terminal Equipment (DTE) Male side of a serial WAN connection.   Wikipedia         

Data-Over-Cable Service Interface Specification (DOCSIS)    Wikipedia         

datagram Basic transfer unit in a packet-switched network.

OSI Layer datagam
Layer 4 segment
Layer 3 packet
Layer 2 frame
Layer 1 Chip (CDMA)

de-encapsulation Process of un-encapsulating packet. Essentially encapsulation in reverse. Layer 2 --> Layer 7.   Wikipedia         

default gateway The IP address of a router that is the exit or "door" to another network. Typically this address is the first or last useable host address for that subnet.   Wikipedia         

default route Where packets go when it does not match a route in the routing table. for IPv4, ::/0 for IPv6.   Wikipedia  Cisco       

default static route a static route with as the destination address. All IP packets that do no have a learned static route will be sent here. This is known a Gateway of Last Resort when configured.            

default-information originate Propagates a default static route in router updates.             

delay (aka: latency) Network quality issue measuring the time required for a packet to travel from the source to the destination.   Wikipedia         

demarcation point (DMARC) Separates customer from service provider equipment.   Wikipedia         


Dense Wavelength-Division Multiplexing (DWDM) Uses optical (fiber) to transmit data over a WAN connection. Multiplies the amount of bandwidth for a single strand fiber. Enables bidirectional communications. 10Gbps multiplexed signal. Multiplex more than 80 channels or wavelengths on single fiber. Supports SONET and SDH.   Wikipedia         

designated port All non-root ports permitted to forward traffic. Selected per trunk.            

Designated Router (DR) uses multicast address for updates.            

Destination IP Address Contains a 32-bit binary value that represents the destination IP address of the packet.            

destination MAC address Part of an Ethernet frame.            

Destination network The address of a remote network and how that network was learned in a routing table.            

Destination Port             

DHCP snooping    Wikipedia         

Dialed Number Identification Service (DNIS)    Wikipedia         

dialup A private WAN infrastructure. Limited to speeds less than 56Kbps due to the PSTN.            

dialup modem    Wikipedia         

Differentiated Services (DiffServ) (aka: soft QoS) Prioritized model to QoS. Marks packets according to the type of service they desire. Most modern QoS configurations are based on the DiffServ approach.   Wikipedia         

Differentiated Services Code Point (DSCP) (formerly: Type of Service (ToS) field) QoS marking that uses the first 6 bits in the DS Byte of an of an IPv4 packet header.   Wikipedia (Differentiated Services)  Wikipedia (IPv4 Header)  Wikipedia (Quality of Service)     

Diffusing Update Algorithm (DUAL) Algorithm used by IGRP and EIGRP. Generates loop-free and backup paths. Developed by Dr. J.J. Garcia-Luna-Achieves at SRI International.   Wikipedia         

Digital Serial Interface (DSI) Protocol for controlling lighting in buildings.   Wikipedia         

Digital Signal 0 (DS0) Dedicated line (not shared) at 64Kbps. 24 bundled is a DS1 line or T1 line.   Wikipedia         

Digital Signal 0 (DS0) Adapter    Cisco         

Digital Signal 1 (DS1) (T1 line) 1.544Mbps bandwidth   Wikipedia  T1Rex       

Digital Signal 3 (DS3) 44.736Mbps   Wikipedia         

Digital Subscriber Line Access Multiplexer (DSLAM)    Wikipedia         

Direct-Sequence Spread Spectrum (DSSS) Spread spectrum modulation technique used to reduce overall signal interference. Modulates data over an entire range of frequencies using a series of symbols called chips which are shorter in duration than a bit. Chips are transmitted at a higher than normall rate and include random data making it difficult to eavesdrop. Supported by 802.11b and 802.11g.   Wikipedia         

directed broadcast Broadcaster sent to all hosts on a specific network such as non-local network. Routers do not forward these by default.            

directly connected network             

directly connected routers             

directly connected static route Static route that uses only the exit interface.            

disabled port Shutdown port.            

Distributed Coordination Function (DCF) Used with CSMA/CA on WLANs to prevent collisions with random back off timers.   Wikipedia         

Distribution Layer Forwards traffic between local networks using Layer 3 devices with ACLs and QoS.   Netacad         

DNS query Request for name resolution.   Wikipedia         

DNS zone transfer Can be either AXFR (full transfer) or IXFR (incremental transfer)   Wikipedia         

domain name Uniquely identifies parts of the internet such as a website. Easier to remember than an IP address.   Wikipedia         

Domain Name System (DNS) (aka: Domain Name Service) (UDP port 53) Translates domain names such as into IP addresses. Originaly published in RFC 882 and RFC 883 November 1983.  Wikipedia  RFC 882  RFC 883  Cisco (Configuring DNS on Cisco Routers)   

Don't Fragment (DF) A router will attempt to fragment a packet that is too big, unless the packet has its don't fragment (DF) bit set. If a packet does exceed an interface's MTU and has its DF bit set, the router drops the packet.            

DROTHER A router that is not a DR or a BDR in an OSPF   Cisco         

duplex settings half duplex and full duplex             

Dynamic Domain Name Service (DDNS) Automatically updates the name server in a domain name system.   Wikipedia         

Dynamic Host Configuration Protocol (DHCP) (RFC 2131) Used to assign an IP address, subnet mask, default gateway, and DNS server address to a host. When a host connects to the network it will broadcast a DHCPDISCOVER message. A DHCP server send a DHCPOFFER back to the host which contains the IP address and subnet mask to be assigned as well as the IP address of the DNS server and default gateway. If multiple offers were received, it will chose one with DHCPREQUEST. The server will send back a DHCPACK to acknowledge the lease is set.

   Wikipedia  YouTube  Cisco     

Dynamic Host Configuration Protocol version 6 (DHCPv6) A method for automatically assigning IPv6 global unicast addresses. Similar to DHCP for IPv4. Can be used along with SLAAC.            

Dynamic NAT (DNAT) (aka: many-to-many) Many inside local user (a network) are mapped to a pool of inside global addresses.            

Dynamic NAT (DNAT)             

dynamic route             

dynamic routing protocol (EIGRP, RIPv2            

Dynamic Trunking Protocol (DTP) Cisco proprietary.   Wikipedia         

E1 Type of dedicated leased line. Contains 32 64Kbps DS0 channels for a total bandwidth of 2.048Mbps.   Wikipedia         

E3 Type of dedicated leased line. Contains 16 DS1 channels for a total bandwidth of 34.4Mbps.   Wikipedia         

edge router Router on the parameter of a network. Three areas to secure: physical security, router hardening, operating system security.            

EIGRP composite metric

K1 (bandwidth) 1
K2 (load) 0
K3 (delay) 1
K4 (reliability) 0
K5 (reliability) 0

EIGRP Convergence neighbors>topology>routing table   Wikipedia         

EIGRP external route Default administrative distance of 170. Shown in a routing table with EX.   Wikipedia         

EIGRP internal route default administrative distance of 90   Wikipedia         

EIGRP IPv4 Messages Have a destination address of and a source address of the outbound interface.   Wikipedia         

EIGRP IPv6 Messages Have a destination all-EIGRP-routes link-local multicast address of FF02::A and the IPv6 link-local address of the exit interface as the source.   Wikipedia         

EIGRP Metric The lowest bandwidth of all the outgoing interfaces between source and destination. Both EIGRP for IPv4 and IPv6 use bandwidth and delay for their composite metric. Reliability and load can also be used to calculate the metric but are not recommended.   Wikipedia         

EIGRP neighbor table Where router adjacencies are stored.   Wikipedia         

EIGRP packets

Acknowledgement Used to acknowledge the receipt of any EIGRP packet. Unreliable and unicast.
Hello Used to discover other EIGRP routers in the network. Unreliable delivery and multicast.
Query Used to request specific information from a neighbor router. Reliable delivery and multicast or unicast.
Reply Used to reply to a query. Reliable and unicast.
Update Convey routing information to known destinations. Reliable and multicast or unicast.

EIGRP successor route Stored in the routing table and the topology table.   Wikipedia         

EIGRP summary route default administrative distance of 5   Wikipedia         

EIGRP topology table Where all learned routes are stored.   Wikipedia         

EIGRP Update packet Convey routing information to known destinations. Reliable and Multicast or unicast.   Wikipedia         

encapsulation When the network layer adds IP header information such as the IP address of the source (sending) and destination (receiving) hosts. After header information is added to the PDU, the PDU is called a packet.   Wikipedia         

Enhanced Data rates for GSM Evolution (EDGE)    Wikipedia         

Enhanced High-speed WAN interface card (EHWIC)    Cisco         

Enhanced High-speed WAN interface card slot (EHWIC slot)    Cisco         

Enhanced Interior Gateway Routing Protocol (EIGRP) Internal gateway, distance vector, classless protocol. Creates and maintains the topology table, neighbor table, and the best path(s) in the routing table. Has an administrative distance of 90 and uses multicast address Protocol number of 88. Uses DUAL. Establishes neighbor adjacencies with hello packets. Can be configured with authentication at the router. Sends out only partial and bounded updates. (Only sends route changes to the routers it affects.) Was originally Cisco proprietary but Cisco released basic functionality to the IETF and it is now an 'open' standard. Introduced in 1993.  Wikipedia  Cisco  RFC 7868     

Enhanced Interior Gateway Routing Protocol for IPv6 (EIGRP for IPv6) EIGRP on IPv6 uses multicast address FF02::A. Uses the all-EIGRP-routers link-local multicast address as the destination address for EIGRP messages. The source and destination addresses are the only difference between EIGRP and EIGRP for IPv6. Made available in Cisco IOS, Release 12.4(6)T.           

enterprise network A large business environment with many users, locations, and systems. Must have support for critical applications, converged network traffic, diverse business needs, and provide centralized administrative control.   Netacad         

equal cost load balancing Two or more paths to a destination with the same metric cost.            

Ethernet    Wikipedia  YouTube       

Ethernet Frame Adds headers and trailers to a Layer 3 PDU. Each section of a frame is called a field. Has no TTL. Two kinds of Ethernet Frames are IEEE 802.3 and Ethernet II.   Wikipedia         

Ethernet II Based on the DIX Ethernet standard.

[ Preamble (8 bits) | Destination Address (6 bits) | Source Address (6 bits) | Type (2 bits) | Data (PDU, 46-1500 bits) | Frame Check Sequence (4 bits) ]


Ethernet LAN interfaces             

Ethernet private line (EPL)    Wikipedia         

Ethernet virtual private line (EVPL)    Wikipedia         

EtherType (0x0800) Two-octet field in an Ethernet frame.   Wikipedia         

Evolved High-Speed Packet Access (HSPA+) Wireless broadband service similar to WiMAX. Maximum data rate is 84Mbps.            

EX An external EIGRP route. AD is 170.   Wikipedia         

Experimental Addresses to are listed as reserved for future use. Could technically be used at a latter date but for now are just for testing.   RFC 3330         

Explicit Congestion Notification (ECN) Part of an IPv4 header.   Wikipedia  Wikipedia (IPv4 Header)       

extended ping Where a source interface or address is specified.            

Extended Service Set (ESS) The combination of 2 or more APs (BSSs) through a common wired DS. Identified with an SSID.   Wikipedia         

Exterior Gateway Protocol (EGP) Used for routing outside an Autonomous System. BGP is the only current EGP and is used by the internet. BGP is often used in place of the name EGP.   Wikipedia         

FE80::/10 IPv6 link local address.   Wikipedia  Cisco       

Feasible Condition (FC) When a neighbors RD is less than this router's FD to the same destination network.            

Feasible Distance (FD) The lowest calculated metric to reach the destination network.            

Feasible Successor (FS) next after successor (backup path)            

FF01::1 All nodes multicast address   IANA         

FF02::2 All routers multicast address   IANA         

FF02::A EIGRP ipv6 multicast   IANA         

Fiber Channel (FC)    Wikipedia         

Fiber To The x (FTTx) Generic term for optical fiber in the last mile of broadband connections. Usually just called Fiber To The Home (FTTH).   Wikipedia         

fiber-optic Type of cable consisting of a thin flexible transparent glass or plastic fiber slightly thicker than a human hair. Can transmit from 100Mbps to 10Gbps for several miles. Uses pulses of light to transmit data making it immune to wiretapping and electrical interference. Two cable types: Single-Mode Fiber (SMF) and Multi-Mode Fiber (MMF). First developed in the 1970s.  Wikipedia         

File Transfer Protocol (FTP) Protocol used for transferring files between systems.   Wikipedia         

FIN (1 bit) Packet sent to both parties to end a TCP three both parties to end a TCP three-way handshake.   Wikipedia         

flag Part of a packet header.            

floating static route Backup route to a static or dynamic route. To test a floating static route, shutdown the main link and the route should appear in the routing table.            

flood guard Limits the number of devices that can connect to a switch port. Once the maximum number of MAC address is exceeded, the port shuts down.   Professor Messer         

flow control Manages the flow of data from the sender to ensure the receiver is not overwhelmed.   Wikipedia  TechTarget       

Flow Label Part of an IPv6 header.   Wikipedia         

Forward Explicit Congestion Notification (FECN) Frame Relay congestion-notification bit   Wikipedia         

forwarding database One of the three OSPF databases. Contains the routing table.            

Forwarding Information Base (FIB) (aka: CAM table)    Wikipedia         

Fragment Offset Field in an IPv4 packet.   Wikipedia         

fragmentation When an intermediate device, usually a router, must split up a packet if it is transferring it to a medium with a smaller MTU.            

Frame Data at Layer 2. Generic frame field:

[ Frame start | Addressing | Type | Control | Data | Error Detection ]


Frame Check Sequence (FCS) Verifies that a frame was received correctly without error.            

Frame encoding technique Converts streams of data bits into a predefined code. Distinguish data bits from control bits. Identify where the frame starts and ends.            

Frame Relay Access Device (FRAD) Router connected to a Frame Relay network.            

Frame Subtype

0x00 Association request frame
0x01 Association response frame
0x02 Reassociation request frame
0x03 Reassociation response frame
0x04 Probe request frame
0x05 Probe response frame
0x08 Beacon frame
0x0A Disassociation frame
0x0B Authentication frame
0x0C Deauthentication frame
   Cisco Support Forums         

Frame Type

0x0 Management Frame
0x1 Control Frame
0x2 Data Frame
   Cisco Support Forums         

Frequency-Division Multiple Access (FDMA)             

Frequency-hopping spread spectrum (FHSS)             

full duplex Two-way commutation. (cell phone)             

fully specified static route Both output interface and the next hop IP address are specified.            

gateway The gateway for a network is the router interface to which the network is directly connected.   Wikipedia         

Gateway Load Balancing Protocol (GLBP) Cisco proprietary protocol similar to HSRP and VRRP but supports load balancing across a max of 4 routers. Up to 1024 virtual routers. Can use clear text or MD5 authentication between group members.    Wikipedia         

Gateway of Last Resort Also known as the default route.   Wikipedia  Cisco       

Gateway to Gateway Protocol (GGP)             

Generic Routing Encapsulation (GRE) Tunneling protocol developed by Cisco. Creates a virtual point-to-point link and encapsulates Layer 3 packets. Has no encryption or authentication of data. By itself GRE is stateless and does not have flow-control. At least 24 bytes of additional overhead for tunneled packets. The original form of VPN. Introduced in 1994 as RFC 1701. Proposed as an IETF standard in 2000 as RFC 2784.  Wikipedia  RFC 1701  RFC 2784  Cisco   

Gigabit Ethernet (GbE) (IEEE 802.3-2008)   Wikipedia         

Global Positioning System (GPS) Global navigation satellite system managed by the United States with 24 total satellites. 3 satellites are needed to determine location while 4 are required to determine location and elevation. Launched February 1978.  Wikipedia         

Globally Unique Identifier (GUID)    Wikipedia         

Gnutella Protocol used for P2P file transfer            

goodput (Throughput - traffic overhead = goodput)            

Gopher port 70            

GPRS Tunneling Protocol (GTP) Group of IP-based communications protocols used to carry general packet radio service (GPRS) within GSM, UMTS and LTE networks.   Wikipedia         

H.323 ITU-T standard for A/V transmission through packets.   Wikipedia         

half duplex Only one device can talk at a time, similar to Nextel push-to-talk. Hubs should only be half duplex.            

Halftone    Wikipedia         

Headend Where signals are first received in a cable network. Data is then distributed downstream to the cable network. Similar to a CO in a telephone network.            

Header Checksum             

hello packet One of the OSPF protocol messages.            

hextet Unofficial term for every 16 bit segment in an IPv6 address   Wikipedia         

Hierarchical Network Design Access layer, Distribution layer, and Core layer.   Wikipedia         

High-Speed Serial Interface (HSSI) developed by Cisco   Wikipedia         

High-speed WAN Interface Card (HWIC)    Cisco         

hold time The amount of time a neighbor should wait before considering the advertising router to be down. Used in EIGRP.            

holddown timer    Wikipedia         

HomeGroup Allows Windwos users to share files across computers on a home network. Introduced with Windows 7  Website  Wikipedia  Netacad     

hop The jump from one outgoing interface on a router to another outgoing interface.            

Hop Limit The amount of hops a packet can take before it is dropped.            

Horizontal cabling Inside Wiring (IW) or plenum that connects the telecommunication room to individual outlets in the wall or floor.   Wikipedia         

horizontal cross-connect Where horizontal cabling connects to a patch panel or punch block.   Wikipedia         

host An end device with a configured IP address.            

Hosts file A text file used by an OS to map IP addresses. Windows filepath: "%WinDir%\System32\Drivers\Etc\Hosts"   Wikipedia  Microsoft Support       

Hot Standby Router Protocol (HSRP) Cisco proprietary protocol.   Wikipedia  RFC 2281  Cisco     

HTTP version 2 (HTTP/2)  Published as RFC 7540 May 14, 2015.  Wikipedia  RFC 7540  IETF     

hub Basic network connectivity device that repeats any signal it receives on one port to all other ports. Operates at OSI layer 1. Succeeded by switches. As of 2011, connecting network segments by repeaters or hubs is deprecated by IEEE 802.3.  Wikipedia         

hub-and-spoke topology Has a WAN link from each remote site when interconnecting multiple sites (for example, multiple corporate locations). Similar to the star topology.            

Hybrid Fiber-Coaxial (HFC) Part of the link to the service provider uses fiber while the other part (usually the last mile) uses coaxial cable.   Wikipedia         

Hypertext Transfer Protocol (HTTP) (port 80) Transfers webpages that make up the World Wide Web (WWW). When a request for a webpage is made to a web server, HTTP specifies the message types used for communication. The three common message types are GET, POST, and PUT. Introduced under RFC 2068 in January 1997.  Wikipedia  RFC 2068  RFC 2616  RFC 7540   

iBook (FireWire)  Released September 13, 2000.  Wikipedia         

IEEE 802.11 Standard for wireless.   Wikipedia         

IEEE 802.11a-1999 (aka: 802.11a) Wireless standard that operates at 5GHz. Maximum transfer speed of 54Mbps. Ratified in 1999.  Wikipedia         

IEEE 802.11ac Wireless standard that operates at 5GHz. Transfer speed of at least 1Gbps. Developed from 2008 to 2013. ANSI approved December 11, 2013. he first 802.11ac products from 2013 are referred to as Wave 1, and the newer higher bandwidth products introduced in 2016 are referred to as Wave 2.  Wikipedia         

IEEE 802.11b-1999 (aka: 802.11b) Wireless standard that operates at 2.4GHz. Maximum transfer speed of 11Mbps. Max range of 802.11b and 802.11g is 32m indoors / 140m outdoors. 802.11b products appeared on the market in mid-1999. The Apple iBook was the first mainstream computer sold with optional 802.11b networking.  Wikipedia         

IEEE 802.11g-2003 Amendment to IEEE 802.11 with transfer speeds up to 54Mbps on the same 2.4GHz band as 802.11b. Max range of 802.11b and 802.11g is 32m indoors / 140m outdoors. Rapidly adopted by consumers starting in January 2003, well before ratification.  Wikipedia         

IEEE 802.11i-2004 (802.11i) Amendment to the original IEEE 802.11 witch improved security. Implemented as Wi-Fi Protected Access II (WPA2). Draft standard was ratified on June 24, 2004.  Wikipedia         

IEEE 802.11n-2009 (aka: 802.11n) Wireless standard that can operate at 2.4GHz or 5GHz and is backward compatible with earlier standards. Maximum transfer rate of 600Mbps. Published October 29, 2009.  Wikipedia         

IEEE 802.11r-2008 (aka: fast BSS transition (FT)) Published July 15, 2008.  Website  Wikipedia       

IEEE 802.16 (WiMAX) Originally published in 2001.  Wikipedia (IEEE_802.16)  Wikipedia (WiMAX)       

IEEE 802.1Q Standard for VLANs over Ethernet. Similar frame format to 802.3. (4 extra bytes fields.) Uses a FCS to verify frames were received. Supports tagged and untagged frames. Untagged frames are placed on the native VLAN.   Wikipedia         

IEEE 802.3 Standard for Ethernet Frame.

[ Preamble (7) | Start of frame delimiter (SFD, 1 bits) | MAC Destination Address (6) | MAC Source Address (6 bits) | 802.1Q tag (optional) (4) | Length (2) | 802.2 Header and Data (46 to 1500) | Frame Check Sequence (FCS, 4 bits) ]

   Wikipedia  Wikipedia (Ethernet Frame)       

in-band router interfaces             

In-Bound Management             

Independent BSS (IBSS) (aka: ad hoc) Formed when two wireless network devices are directly connected without an AP. For example; two laptops directly connected wirelessly to transfer files.   Wikipedia         

infinite ping ping -t             

infrastructure mode A wireless LAN topology. Wireless devices are connected using another device such as an access point or a wireless router.            

ingress Incoming/entering.            

ingress filtering Ensures packets come from where they claim.   Wikipedia         

Integrated Broadband Services (IBBS)    Wikipedia         

Integrated Services (IntServ) (aka: hard QoS) Parameterized approach to QoS that can make strict bandwidth reservations. Applications use Resource Reservation Protocol (RSVP) to request and reserve resources through a network.   Wikipedia         

Integrated Services Digital Network (ISDN) Private, circuit-switched, WAN infrastructure. Uses telephone circuits to send data.   Wikipedia         

Integrated Services Routers (ISRs)    Wikipedia         

Interface ID Part of an IPv6 global unicast address. The 'host' portion of the address. A single host may have multiple interfaces each with one or more IPv6 addresses.            

Interior Gateway Protocol (IGP) Used for routing within an Autonomous System. (RIPv1, RIPv2, IGRP, EIGRP, OSPF, IS-IS)            

Interior Gateway Routing Protocol (IGRP) A link-state routing protocol.            

International Telecommunication Union (ITU) Defines open standards and protocols used by the data link layer.   Wikipedia         

internet A network of networks.   Wikipedia  YouTube (Warriors of the Net)       

Internet Architecture Board (IAB)    Website  Wikipedia       

Internet Assigned Numbers Authority (IANA) Assigns port numbers.   Wikipedia  Website       

Internet Control Message Protocol (ICMP) Used by diagnostic tools such as ping and traceroute. Defined in RFC 792 September, 1981.  Wikipedia  RFC 792       

Internet Control Message Protocol version 6 (ICMPv6) ICMP for IPv6            

Internet Corporation for Assigned Names and Numbers (ICANN)    Wikipedia  Website       

Internet Engineering Steering Group (IESG)    Website  Wikipedia       

Internet Engineering Task Force (IETF)             

Internet Group Management Protocol (IGMP)             

Internet Header Length (IHL)             

Internet Key Exchange (IKE) Phase 1 Authentication. 3 keys exchanged. Main mode or aggressive mode.            

Internet Key Exchange (IKE) Phase 2 Key Exchange. IPsec transform sets. unidirectional IPsec SAs.            

Internet Message Access Protocol (IMAP) (IMAP: port 143, IMAPS: port 993) Protocol for email retrieval. IMAP Secure (IMAPS) operates over SSL or TLS. Designed by Mark Crispin in 1986.  Wikipedia         

Internet Printing Protocol (IPP)  Proposed by Novell in 1996.  Wikipedia         

Internet Protocol (IP) The network layer service implemented by the TCP/IP protocol suite. (Connectionless, Best Effort (unreliable), Media Independent.)   Wikipedia  YouTube       

Internet Protocol Flow Information Export (IPFIX)             

Internet Protocol Security (IPsec) Authentication Guaranties message comes from the right person and was not forged. (PSK, RSA)            

Internet Protocol version 4 (IPv4) (FC 791) In use since 1983 when it was deployed on the Advanced Research Projects Agency Network (ARPANET). An example address would be:            

Internet Protocol version 6 (IPv6) An IPv6 address is 128 bits long over IPv4's 32 bits. Every 4 bits is represented by a single hexadecimal digit; for a total of 32 hexadecimal values. Not case sensitive. Network portion identified by a prefix (/64). Can range from /0 to /128. Has three address types: Unicast, Multicast, and Anycast. Three address scopes: global, unique-local, and link-local. Draft standard in RFC 2460 December 1998. Standardized in RFC 8200 July 14, 2017.  Wikipedia  RFC 2460  RFC 8200  Internet Society   

Internet Relay Chat (IRC)    Wikipedia         

Internet Service Provider (ISP) Provide access to the internet. Four things to consider when choosing an ISP:

   Wikipedia  Netacad       

Internet Systems Consortium (ISC)    Wikipedia         

Internet-Draft (I-D) Working documents published by the IETF.   Wikipedia  IETF       

Internetwork Operating System (IOS) Operating System that runs on Cisco networking gear. Can be stored on non-volatile flash a TFTP server. Copied to RAM when device is powered on.   Wikipedia  YouTube       

Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX)    Wikipedia         

Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) An IPv6 transition mechanism meant to transmit IPv6 packets between dual-stack nodes on top of an IPv4 network.   Wikipedia         

intranet internal network of LANs and WANs   Wikipedia         

Intrusion Detection System (IDS) Monitors network traffic. Developed by SRI International in 1984.  Wikipedia         

Intrusion Prevention System (IPS) Responds immediately and inspects traffic before it enters the network. Developed during the late 1990s.  Wikipedia  YouTube       

Inverse Address Resolution Protocol (InARP)             

IOS update

   YouTube (Lextraindotcom)         

IP Private Branch Exchanges (PBXs)             

IP Suite A suite of protocols required for sending and receiving data using the Internet.   Wikipedia         

IP-Header Identifies the packet characteristics. Significant fields in the IPv4 header include: Version, Differentiated Services (DS), Time-to-Live (TTL), Source IP Address, Destination IP Address.            

IPv4 Addresses IPv4 includes, private, network, broadcast, loopback, link-local, TEST-NET, and experimental addresses.            

IPv4 embedded Type of unicast address used to help transition from IPv4 to IPv6.            

IPv4 packet Contains two parts; the IP Header and the Payload. Does not have subnet mask.            

ipv6 address autoconfig Router(config-if)# ipv6 address autoconfig   Cisco         

IPv6 anycast Unicast address that can be assigned to multiple devices. Packets are routed to the nearest device with that address.            

IPv6 default static route prefix is ::/0            

IPv6 global unicast A type of IPv6 unicast address. Globally unique and routable (like public IPv4). Current ranges are 2000::/3. Can be configured manually or automatically with SLAAC and DHCPv6. Has a global routing prefix subnet ID and interface ID.

[ global routing prefix, 48 bits | subnet ID, 16 bits | Interface ID, 64 bits ]


IPv6 multicast For sending an IPv6 packet to multiple destinations.            

IPv6 unicast Uniquely identifies an interface. These can be Global Unicast, Link-Local, Loopback, Unspecified Address, Unique Local, Embedded IPv4.            

IS-IS IGP, link-state, classless protocol.   Wikipedia         

ISDN BRI Two 64Kbps B channels and one 16Kbps D channel. Used for home and small enterprise. Inadequate for video but works fine for voice.            

ISDN PRI 23 64Kbps B T1 (30 B E1) channels and 1 64Kbps D channel (1.544Mbps total).             

jitter Network quality issue where there is an uneven arrival of packets.            

K9 Branding for Cisco IOS with security features.   Cisco Learning Network         

key size (key length) measure in bits. Large keys can degrade performance but are more secure.   Wikipedia         

key space Possibilities that can be generated by a specific key length.   Wikipedia         

L Local route in a routing table.            

Label Switch Router (LSR) MPLS element that makes frame-forwardig decisions based on lables applied to frames.            

last mile    Wikipedia         

Latin America and Caribbean Network Information Centre (LACNIC) Regional Latin-American and Caribbean IP address registry.   Wikipedia  LACNIC       

Layer 1 (physical layer) Transmits Bits across local media.   Wikipedia         

Layer 2 (data Link layer.) Packages Layer 3 PDUs into frames. Manages access to frames and network media.   Wikipedia         

Layer 2 address (physical or MAC address) 48 bits long, 6 octets, 12 hexadecimal digits. The first 6 hex numbers indicate the manufacture of the NIC (OUI), the last 6 are serial number of the NIC.            

Layer 3 network layer of the OSI model.   Wikipedia         

layer 3 address             

Layer 4 Transport layer (TCP, UDP)   Wikipedia         

Layer 4 Switching Used to filter TCP and UDP packets before they reach Layer 3. HP proprietary.            

Layer 5 session layer   Wikipedia         

Layer 6 presentation layer    Wikipedia         

Layer 7 (application layer)   Wikipedia         

Layer 8 user error   Wikipedia         

leased line Private WAN infrastructure. Dedicated point-to-point connection leased from a service provider. Bandwidth is not shared with multiple customers. Examples include: T1, E1, T3, and E3. Can use multiplexing to transmit multiple conversations over different 64Kbps channels. Terminates on a CSU/DSU device.   Wikipedia         

level 1 parent route a subnetted level 1 network route. Not an ultimate route.            

level 1 route routing table entry with a subnet mask less than or equal to the classful mask of the network address. (Network route, Supernet route, Default route)            

level 2 child route a route that is a subnet of a classful network address.            

Lightweight Access Point Protocol (LWAPP)             

limited broadcast Broadcasts only to hosts on the local network (broadcast domain). Always use destination address o Will not be forwarded by a router.             

type 1 Router LSA. Also known as router link entries. All routers generate type 1 LSAs. Flooded only inside the area they came from, never go beyond ABR. Link ID is the router ID of originating router.
type 2 Network LSA. Only exists for multi-access and nonbroadcast multi-access. Gives routers information about multi-access networks in the same area. Identify the routers and network addresses of multi-access links. Only generated by a DR.
type 3 Summary LSAs. Advertises networks from other areas. ABR send type 3 LSAs to other areas. Routes are not summarized by default. Link state ID identified by network address.
type 4 Advertise external networks into an OSPF routing domain. Only generated by ABRs. Identifies the ASBR and provides a route to it.
type 5 AS external LSAs. Most summarized by default.
type 6 Multicast OSPF LAS
type 7 Defined for NSSAs
type 8 External attributes LSA for BGP
type 9 Opaque LSAs
type 10 Opaque LSAs
type 11 Opaque LSAs

load balancer (aka: content switch) distributes incoming requests accross multiple servers. All servers contain the same data.   Wikipedia         

Local Area Network (LAN) A network that is in a small geographical area such as home or office.   Wikipedia         

Local Connector (LC) (aka: little connectors) Mini Form Factor (MFF) fiber-optic connector popular for use with Fiber-Channel adapters, fast SANs, and Gigabit Ethernet. Developed by Lucent Technologies.  Wikipedia         

local default route             

local host Administrative distance is 0.            

Local Loop The physical cable that connects the CPE to the CO of a service provider. (The last mile.)            

Local Management Interface (LMI)    Wikipedia         

local network             

local network route             

logical Address (OSI Network Layer 3)     Wikipedia       

loopback address Can ping to see if NIC is working. Is also a special address a host can use to direct traffic to themselves. Additional loopback addresses include through for IPv4. IPv6 also has ::1/128 and just ::1 for loopback addresses.   Wikipedia  PC Magazine       

MAC Address Table Overflow             

MAC database instability When the same broadcast frames are received on a switch, an endless loop can occur. Ethernet frames do not have TTL so they are constantly forwarded, confusing the MAC address table.   Wikipedia         

Mail Delivery Agent (MDA) Accepts mail form the MTA and preforms the actual delivery.   Wikipedia         

Mail Exchange (MX) DNS record.   Wikipedia         

Mail Transfer Agent (MTA) Used to forward email   Wikipedia         

Mail User Agent (MUA) An email client.   Wikipedia         

Management Information Base (MIB) SNMP component that defines the structure of SNMP management objects. The structure of these management objects is defined by a managed device's MIB. Interfaces and their details, such as errors, utilization, discards, packet drops, resets, speed and duplex, system memory, utilization of bandwidth, storage, CPU, and memory, are able to be monitored and reported via SNMP.   YouTube (SolarWinds)         

management ports Ports used for managing a network device.

AUX port
     PC Mag       

Marking QoS mechanism that alters bits within a frame, cell, or packet to indicate how the network should treat that traffic.            

Maximum Transmission Unit (MTU) The maximum size of the PDU each medium can transport.            

maximum-paths Paths that should be maintained for load balancing.            

Media Access Control (MAC) Sublayer of OSI Layer 2.   Wikipedia         

Media Access Control address (MAC address) (aka: burned-in address, Ethernet hardware address (EHA), hardware address or physical address (not to be confused with a memory physical address)) Unique identifier assigned to a network interface controller (NIC) for communications at the data link layer of a network segment.   Wikipedia         

Medianet Cisco borderless service. Helps deploy and manage media applications. Metadata   YouTube  Cisco       

metric Quantitative value used to measure the distance to a given network in a routing table. Lower values indicate preferred routes.   Wikipedia         

Metro Ethernet (aka: metropolitan-area Ethernet, Ethernet MAN) MAN based on Ethernet. Packet-switched WAN.   Wikipedia         

modem Translates digital signals to analog.   Wikipedia         

Multi-Mode Fiber (MMF) (aka: multimode fiber) Most often used in horizontal cabling. Can transmit up to 10Gbps for up to 550 meters (1,804 feet) or just over one-third of a mile.   Wikipedia         

multiaccess network             

Multiaccess OSPF             

Multiarea OSPF             

multicast (one to many) Sending a packet from one host to a selected group of hosts, possibly in different networks. Saves bandwidth by only sending one copy of the traffic. IPv4 addresses are used for multicast addresses. reserved for link-local reserved for globally scoped addresses

multicast Domain Name System (mDNS) resolves hostnames in networks without a local name server.   Wikipedia         

multiple frame transmission Multiple copies of unicast frames are sent to the destination.            

Multiple Input, Multiple Output (MIMO) Achieves higher throughput by using multiple antennas to transmit and receive. Found in IEEE 802.11n and above.    Wikipedia         

Multiple Spanning Tree Protocol (MSTP)             

Multiplexer (MUX)             

multiplexing Process where hosts can support multiple simultaneous sessions over a single link.   Wikipedia         

Multiprotocol Label Switching (MPLS) A packet-switched WAN. Can be used to deliver any type of packet over a WAN. Elements include; Customer Premises Equipment (CPE), Customer Edge (CE), Edge Label Switch Router (ELSR), Provider Edge (PE), Label, Switch Router (LSR), and Provider (P) routers. Header is 32 bit long, which contains the label.    Wikipedia  YouTube       

NAT translations (xlates)    Netacad         

Neighbor Discovery (ND)             

Neighbor Discovery Protocol (NDP)    Wikipedia         

Neighbor Discovery Protocol (NDP)             

Neighbor Solicitation (NS) ND message            

neighbor table Table in OSPF adjacency database. Lists neighboring routers. Can be viewed on IOS with show ip ospf neighbor.            

Network Access Control (NAC) Approach to endpoint security.   Wikipedia         

Network Access Server (NAS) Also known as an access server.   Wikipedia         

network address    Wikipdedia         

Network Address Translation (NAT) Remaps one IP address space into another. Three kinds of NAT: static, dynamic, and overload (PAT). Translation process:

Inside local private IP address being translated
Inside global public IP address referencing an inside device
Outside local private IP address referencing an outside device
Outside global public IP address referencing an outside device

 One of the first NAT appliances is the PIX firewall developed in 1994.  RFC 1631  Wikipedia (Network Address Translation)  Wikipedia (Cisco PIX)  YouTube (VambarInc)  How Stuff Works 

Network Address Translation 64 (NAT64) Similar to NAT for IPv4 but used for IPv6.   Wikipedia         

Network Administration Control (NAC) Cisco's version of Network Access Control.   Wikipedia         

Network Based Application Recognition (NBAR)    Cisco         

Network Basic Input/Output System (NetBIOS) Works at the OSI session layer (layer 5). Developed as an API.   Wikipedia         

Network Control Protocol (NCP)             

Network Discovery Protocol (NDP) Five main NDP messages.

Router Solicitation (RS)
Router Advertisement (RA)
Neighbor Solicitation (NS)
Neighbor Advertisement (NA)

Network Interface Controller (NIC) (aka: network interface card, network adapter, LAN adapter, physical network interface, NIC card) Connects a computer to a network.   Wikipedia         

Network Interface Device (NID) Demarcation point between the carrier's local loop and the customer's premises wiring   Wikipedia         

Network Layer (OSI Layer 3) Responsible for packet forwarding including routing through intermediate routers. Addresses data messages and translates logical addresses into physical ones.   Wikipedia         

Network Layer Protocols IPv4 and IPv6 are the current protocols. IPX, AppleTalk, and CLNS/DECNeT are some other legacy protocols not often used.            

Network Load Balancing (NLB) (aka: dual-WAN routing or multihoming) Distributing the workload across multiple paths to provide fault tolerance and improve performance.   Wikipedia         

Network Management Protocol (NMP)             

Network Operations Center (NOC) (aka: Network Management Center) Location used to monitor and control a network.   Wikipedia         

network route routing table entry that has a subnet mask equal to that of the classful mask.            

Network Time Protocol (NTP) (UDP port 123) Synchronizes time across devices in the network. Helps when comparing syslog.   Wikipedia  Debian Wiki       

Network-Aware Applications Can use protocols to communicate with lower level protocols on the OSI and TCP/IP models. Examples include web browsers and Email clients.            

next hop The address of the next router interface.            

Next Hop Resolution Protocol (NHRP)             

nibble 4 bits (half a byte)   Wikipedia         

node    Wikipedia         

node port (N_Port)    Wikipedia         

Non-existent Domain (NXDOMAIN)    DNS Knowledge         

nonrepudiation (digital signature) Similar to authentication. Cannot refute or "repudiate" the validity of a message.   Wikipedia         

NT1 ISDN device that interconnects a 4-wire ISDN circuit with a 2-wire ISDN circuit            

null0 ( The bit bucket. Packets that don            

O In a routing table, 'O' indicates a route that was learned dynamically by OSPF. Learned by type 1 and 2 LSAs.   Cisco         

O E1 External routes to an AS. Learned by type 5 LSAs   Cisco         

O E2 External routes to an AS. Learned by type 5 LSAs   Cisco         

O I    Cisco         

O IA OSPF Interarea (originated from another area) summary LSAs. Learned from type 3 or type 4 LSAs.   Cisco         

Object Identifier (OID)    Csico  YouTube       

Object Linking and Embedding (OLE) Microsoft protocol.   Wikipedia         


Open Shortest Path First (OSPF) (IGP, link-state, classless protocol) Efficient trigger updates. Fast convergence. Easily scalable.    Wikipedia         

Open Shortest Path First version 3 (OSPFv3) UIGP, link-state, classless protocol. uses Dijkstra's algorithm to make IPv6 routes. Very much like it's IPv4 counterpart, OSPF.            

Open Systems Interconnection model (OSI model)

layer 1 physical
layer 2 data link
layer 3 network
layer 4 transport
layer 5 session
layer 6 presentation
layer 7 application
   Wikipedia  YouTube (Eli the Computer Guy)       

OpenSSH An open source version of SSH typically used on Linux systems.   Wikipedia  Website       

Optical Carrier 3 (OC3) 155.52 Mbps bandwidth leased line.   Wikipedia         

Optical Time Domain Reflectometer (OTDR) Tool that enables you to locate a break in fiber-optic cable.   Wikipedia         

Orthogonal Frequency-Division Multiplexing (OFDM) Method of encoding digital data on multiple carrier frequencies. Uses a relatively slow modulation rate for symbols with high data rates. Simultaneous transmission of data over 52 data streams. Supports Supported by 802.11a, 802.11g, and 802.11n.   Wikipedia         

OSPF area A group of routers that share the same link-state information in their link-state databases.   Wikipedia         

OSPF message types Used to discover routers and exchange routing information. Packets: Hello, DBD, LSR, LSU, LSAck. Transmitted over IP   Wikipedia         

OSPF priority Can be a number 1-255. (255 is the highest and is most likely to become a DR.) The DR or BDR is chosen by the highest priority value. A priority of 0 does not become a DR/BDR.DR/BDR.   Wikipedia         

OSPF route calculation In an OSPF network, all routers calculate the best path to destination within their own area, then the internetwork, and then to external destination networks.   Wikipedia         

OSPF states

Down state: No Hello packets are received.
Init state: Hello packets received.
Two-way state: DR and BDR election.
ExStart state: Negotiate master / slave and DBD packet sequence number.
Exchange state:Exchange of DBD packets
Loading state: Additional information is sent.
Full state: Routers converged.

outgoing interface The exit interface used when forwarding packets to a destination network.            

packet Data at OSI Layer 3.    Wikipedia         

packet buffer             

packet capture (PCAP)    Wikipedia         

Packet Filter (PF)    Wikipedia         

packet length    Wikipedia         

packet sniffer Monitors packets through the network.            

Packet-Switched Network Files are split into many pieces before transmitting and may traverse multiple paths to reach the destination. Frame Relay and ATM are examples of a packet-switched WAN.   Wikipedia         

packets per second (pps) Measurement of throughput.   Wikipedia         

parallel Can send multiple bits of information over multiple wires at the same time to theoretically reach the destination faster.   Wikipedia         

parity Indicates whether a number is even or odd.   Wikipedia         

parity bit    Wikipedia         

parity file    Wikipedia         

parity flag    Wikipedia         

partial mesh Network topology that balances redundancy with easy management and cost effectiveness.            

passive interface Interface that is not receiving updates but can send them. Configured with 'passive-interface' and disabled with 'no passive-interface'.            

password recovery

   Cisco (Password Recovery Procedures)  Cisco (Catalyst 2960 and 2960-S Software Configuration Guide, 12.2(53)SE1 - Troubleshooting)       

patch panel    Wikipedia         

payload Contains the Layer 4 segment information and the actual data.            

Peer-to-Peer (P2P) Type of network connection. Peers share information without the need of a central server. All devices are technically servers.   Wikipedia         

Per VLAN Spanning Tree (PVST)    Wikipedia         

Permanent Virtual Circuit (PVC) (aka: Private Virtual Circuit). Used to carry both voice and data traffic. Preconfigured by the carrier and only operate in DATA TRANSFER and IDLE modes. Supports data rates up to 4Mbps. More common than SVCs.   Wikipedia    Netacad     

physical address (aka: Layer 2 address or MAC address) Used to identify devices at the physical layer (Layer 2). Allows devices to communicate on the same local network.   Wikipedia         

Physical Layer (OSI Layer 1) The first and lowest layer. Transmits raw bits from one computer to another and regulates the transmission stream over a medium. This layer may be implemented by a PHY chip.   Wikipedia         

piconet Ad hoc network that links a wireless user group (WUG) of devices using Bluetooth technology protocols. Consists of two or more devices occupying the same physical channel. Allows one master device to interconnect with up to seven active slave devices.   Wikipedia         

ping Network utility used to test network connectivity between devices. Can also be used to test DNS. Example: ping Written by Mike Muuss in December 1983.  Wikipedia         

ping sweeps    Wikipedia         

Plain Old Telephone Service (POTS)             

Point-to-Point Frame    Wikipedia         

Point-to-point network Two routers connected over a common link.   Wikipedia         

Point-to-Point protocol Data link layer protocol. Establishes logical connection (sessions) between two nodes (interfaces).            

Point-to-Point Protocol (PPP) Common non-proprietary Layer 2 protocol found on dedicated leased lines. Sends packets over point-to-point links. Offers features such as: multilink interface, looped link detection, error detection, and authentication. Supports PAP and CHAP authentication. Can be configured on interfaces with asynchronous serial, synchronous serial, HSSI, and ISDN. Defined in RFC 1661 July 1994.  Wikipedia  RFC 1661       

Point-to-Point Protocol over Ethernet (PPPoE) Encapsulating protocol for PPP. Typically used with DSL.   Wikipedia         

pointer record (PTR) Type of DNS record that maps an IP address to a hostname. Defined in RFC 1035 November 1987.  Wikipedia  RFC 1035       

Policy-based detection             

Polyalphabetic cipher Any cipher based on substitution including The Vigen   Wikipedia         

polyalphabetic key shift             

port Identifies which application data will go to. 0-1023 are well known ports, 1024-49151 are registered ports, 49152-65535 are private and/or dynamic ports. Several ports include:

Well Known Ports
port 0 (Legacy) 768 bit key.
port 21 FTP
port 22 SSH
port 23 Telnet
port 25 SMTP
port 53 DNS
port 80 HTTP
port 110 POP3
port 143 IMAP
port 161 SNMP
port 194 IRC
port 443 HTTPS
port 520 RIP
port 531 Chat (AOL Instant Messenger)
Registered Ports
port 1433 MS SQL
port 1812 RADIUS authentication protocol.
port 1863 MSN Messenger, TCP
port 2000 Cisco SCCP (VoIP), TCP
port 2948 WAP (MMS)
port 5004 (RTP) Voice and video transport protocol.
port 5060 SIP (VoIP)
port 8008 Alternate HTTP
   IANA  Wikipedia (Port 0)       

port 465 (SMTPS)   Wikipedia  Stack Overflow       

Port Address Translation (PAT) Many-to-one address mapping. An extension of NAT also known as NAT overload.   YouTube         

Port Aggregation Protocol (PAgP) Cisco proprietary protocol for EtherChannel. Modes: On - forces the interface to channel without PAgP. PAgP desirable - interface in active negotiating state. PAgP auto - interface in passive negotiating state            

port mapping             

Port Security Prevents MAC address spoofing and MAC table overflows. Can statically assign or dynamically learn MAC addresses to permit.   Cisco         


Post Office Protocol (POP) (v2, port 109. v3, port 110.) Application layer protocol used for receiving email. First defined in RFC 918 October 1984.  Wikipedia  RFC 918       

Post Office Protocol version 2 (POP2)  Defined in RFC 937 February 1985.  Wikipedia  RFC 937       

Post Office Protocol version 3 (POP3)  Defined in RFC 1081 November 1988.  Wikipedia  RFC 1081       

Power over Ethernet (PoE) (802.3af)            

PPP frames

{ Flag | Address | Control | Protocol | Data | Frame Check Sequence (FCS) }

Preamble Start of an Ethernet frame.   Wikipedia         

prefix Number of network bits in the subnet mask. /24=            

Presentation Layer (OSI Layer 6) Translates application layer data to an intermediate form that provides security, encryption, and compression for the data.   Wikipedia         

private address Private IPv4 address ranges are: - ( - ( - (
   Wikipedia  RFC 1918       

Private and/or Dynamic Ports 49152-65535            

Private Branch Exchange (PBX)             

Private Virtual Circuit (PVC) (aka: Permanent Virtual Circuit (PVC)   Wikipedia         

Private VLAN (PVLAN) Edge (protected ports)   Cisco         

private WAN infrastructure Dedicated (Leased Lines) and Switched (Circuit or packet switched) WANs.            

promiscuous mode Where a network adapter is set to captures packets.            

Protocol Protocols provide the rules and formats that govern how data is treated.            

Protocol Data Unit (PDU) The form that a piece of data takes at any layer.            

protocol field Part of a packet header represented by a number to identify the protcol being used in the PDU. 6 is used to indicate TCP, 17 for UDP, 88 for EIGRP, 89 for OSPF.            

Protocol-Dependent Module (PDM) Allows EIGRP to route for various Layer 3 protocol packets. (IPv4 and IPv6)   Wikipedia         

Provider Edge (PE) MPLS element that is the provider's router connecting the customer.   Wikipedia         

Public Switched Telephone Network (PSTN) A circuit-switched WAN.            

public WAN infrastructure Internet. (Broadband, VPN)            

PVC status

DELETED PVC is not present and no LMI information is being received
INACTIVE Connection to remote host is unavailable
STATIC LMI is disabled on that interface

Quality of Service (QoS) Suite of technologies that allows you to strategically optimize network performance for select traffic types. Commonly used QoS mechanisms include:

Congestion management
Congestion avoidance
Policing and shaping
Link efficiency

Two principal approaches to QoS in modern packet-switched IP networks:
Integrated Services (IntServ)
Differentiated Services (DiffServ)

R In a routing table, 'R' indicates a network that was learned dynamically using RIP.            

Random Early Detection (RED) (aka: random early discard, random early drop) QoS tool that seeks to avoid congestion before it becomes an issue on the network.   Wikipedia         

Rapid PVST+ The Cisco implementation of RSTP on a per-VLAN basis.     Netacad       

Rapid Spanning Tree Protocol (RSTP) IEEE 802.1w evolution of the original STP 802.1D standard.   Wikipedia         

Rapid STP (RSTP) (802.1w)     Cisco       

Real-Time Messaging Protocol (RTMP) (port 1935)   Wikipedia         

Real-time Transport Protocol (RTP or RTTP)    Wikipedia         

Received Signal Strength Indicator (RSSI) Measures the power of a wireless signal.            

recursive static route A static route that uses an IP address as the next hop rather than an exit interface.   Cisco         

redistribute When two networks running different routing protocols need to communicate their route can be redistributed. Static routes ( quad zero) can be used for connecting to ISPs.      Cisco       

reference model For networking, it clarifies the processes of networking and makes troubleshooting easier. Examples include TCP/IP and OSI.            

Regional Internet Registries (RIRs) RIRs are responsible for allocating IP addresses to ISPs.            

Registered Jack (RJ) (ISO 8877) Standard connector used in data and telecommunication networks.   Wikipedia         

Registered Ports 1024-49151            

Reliable Transport Protocol (RTP) Used in EIGRP. (TCP)   Wikipedia         

Remote Desktop Protocol (RDP) (TCP port 3389 UDP port 3389) Proprietary Microsoft protocol.   Wikipedia         

remote host Located on a remote network and have different network addresses.            

remote network Network that originate from another router and have different network addresses.            

Remote Network Network/packet mode   Microsoft TechNet  Netacad       

remote network entries

{ Router source | Destination network | [Administrative distance/Metric] | Next-hop | Route timestamp | Outgoing interface }


Remote Office / Branch Office (ROBO)             

remote route Entry in a routing table that came from a remote network. Can be configured manually or learned dynamically.            

repetition control structure (aka: iteration control structure) Series of statements in a loop.            

reset (RST) TCP flag.            

Resource Reservation Protocol (RSVP) QoS IntServ mechanism that reserves resources for an integrated services internet.  Operates at OSI Layer 4 (transport). Defined in RFC 2205 September 1997.  Wikipedia  RFC 2205       

Reverse Address Resolution Protocol (RARP) Ethernet type 0x8035.            

Riser Cabling (Also known as Vertical cabling)    Wikipedia         

RJ-45 plug The standard connector used for LANs on Ethernet cable.            

Rollover Cable (Console Cable) T658A to reversed T658A.   Wikipedia         

root bridge Bridge with the smallest (lowest) bridge ID. Switch in STP that determines which redundant paths to block. The root bridge switch can be set with a priority of 0. If multiple devices have a priority of 0, it will chose the one with the lowest MAC address.   Wikipedia         

root hints Resolve queres for zones that don't exist on the local DNS server. Only use if forwaders are not configured or fail to respond   YouTube (Microsoft MCSA/MCSE Learning Channel)         

root port Switch ports closest to the root bridge            

Round Robin (RR) Network scheduling algorithm.   Wikipedia         

route poisoning    Wikipedia         

route resolvability The process used by the routing table to determine the exit interface used to forward a packet.            

route source How a route was learned in a routing table. Usually represented by the first letter of the routing protocol or the kind of connection. (i.e. R for a RIP route, C for a directly connected route, S for a static route...)            

route summarization (route aggregation) advertising a contiguous set of addresses as a single address with a less-specific, shorter subnet mask.            

route timestamp When a route was last heard in a routing table.            

router routes packets across different networks and to other devices on the current network.   Wikipedia         

Router Discovery Protocol (RDP)             

router ID Both IPv4 and IPv6 use the same 32-bit router ID in EIGRP. Can be configured with the            

router on a stick Allows hosts logically separated by a VLAN to communicate.   Wikipedia         

Router Solicitation (RS) When configured with SLAAC, a host will send an RS message to the router.            

routing Choosing the best path through a network.   Wikipedia         

Routing Header (RH)             

Routing Information Base (RIB) Also known as the routing table.   Wikipedia         

Routing Information Protocol (RIP) (Administrative Distance: 120, UDP port 520) Distance-vector routing protocol that uses hop count as a routing metric. Has a maximum hop count of 15. Implements the split horizon, route poisoning and holddown mechanisms to prevent incorrect routing information from being propagated. Uses multicast address   Wikipedia         

Routing Information Protocol (RIPng) An extension of RIPv2 that supports IPv6.   Wikipedia         

Routing Information Protocol Version 2 (RIPv2) Uses UDP through port 520. Broadcast    Wikipedia         

routing loop Where duplicate frames are sent across the network to the same router and get stuck in a loop.   Wikipedia         

routing protocol How routers communicate with each other. A set of processes, algorithms, and messages used to exchange routing information and populate the routing table with the best path.   Wikipedia         

routing table Each router or computer has a unique routing table of where to send packets. Volatile. Table in the OSPF forwarding database. [Route source | Destination network | Outgoing interface] Can be shown using route print on Windows, route -n on Linux, and show ip route on Cisco IOS.   Wikipedia  Microsoft TechNet       

running configuration file Current configuration in Cisco IOS            

S Identifies a static route created to reach a specific network in a routing table.            

Second Level Address Translation (SLAT) (aka: nested paging) Hardware-assisted virtualization technology.   Wikipedia         

Secure Device Manager (SDM) Management software for Cisco routers            

Secure Shell (SSH) A more secure form of telnet that uses port 22 instead of port 23 as in the case of telnet.   Cisco  Pluralsight blog       

Secure Shell (SSH) Configuration Can be configured on a Cisco device with a;

hostname carter
aaa new-model
username cisco password 0 cisco
ip domain-name
crypto key generate rsa
ip ssh time-out 60
ip ssh authentication-retries 2
line vty 0 4
transport input SSH



Segmentation Dividing a data stream into small pieces.            

sequence control structure Series of statements that follow one another.            

Sequence Number             

serial Sends information in a single stream, one at a time. (bidirectional)            

Serial Line Internet Protocol (SLIP) standard protocol for point-to-point serial connections using TCP/IP   Netacad         

serial WAN interfaces             


service provider             

service sequence-numbers Service on networking devices that puts sequence numbers in syslog messages.            

Service Set    Wikipedia         

Service Set Identifier (SSID) (aka: BSSID) Associated with the MAC address on a wireless Access Point (AP). Identify logical networks (including extended service sets) and serve as the "network name".   Wikipedia         

Serving Area Interface (SAI) Outdoor enclosure or metal box that allows access to telecommunications wiring.   Wikipedia         


Session Initiation Protocol (SIP) (port 5060)   Wikipedia  YouTube       

Session Layer (OSI Layer 5) Establishes and controls data communication between applications operating on two computers.   Wikipedia         

Shielded Twisted Pair (STP) A type of twisted pair cabling with a layer of braided foil shielding surrounding the wires to reduce electrical interference.   Wikipedia         

Shortest Path First (SPF)             

signal-to-noise ratio (SNR)    Wikipedia         

Simple Mail Transfer Protocol (SMTP) (TCP port 25) Transfers mail messages and attachments.   Wikipedia         

Simple Mail Transfer Protocol Secure (SMTPS) port 465   Wikipedia         

Simple Network Management Protocol (SNMP) Service uses port 161, trap uses port 162. Uses SNMP managers, agents, and MIBs. Consumes more bandwidth than Syslog. First defined in RFC 1067 August 1988.  Wikipedia  RFC 1067  Cisco (Configuring SNMP)  Netacad  CCIE or Null! 

Simple Network Management Protocol version 1 (SNMPv1) Community-based form of security. Able to access an agent's MIB as defined by ACLs and passwords. Offer minimal security features. (No encryption or authentication.) Defined in RFC 1157 May 1990.  Wikipedia  RFC 1157       

Simple Network Management Protocol version 2c (SNMPvc2) Community-based form of security. Able to access an agent's MIB as defined by ACLs and passwords. Offer minimal security features. (No encryption or authentication.)   Wikipedia         

Simple Network Management Protocol version 3 (SNMPv3) Authenticates and encrypts packets between management devices. Defined in RFC 5590 June 2009.  Wikipedia  RFC 5590       

Simple Service Discovery Protocol (SSDP)  October 28, 1999  Wikipedia  IETF       

single area OSPF Uses area 0. Good for small campus. Has a large routing table because summarization is disabled by default. LSDB. Frequent SPF algorithm calculations.            

Single-Mode Fiber (SMF) Commonly used as backbone cabling or cable in phone systems. Light travels straight down the fiber and does not bounce off the fiber walls. Can transmit 10Gbps up to 40 kilometers (25.85 miles).   Wikipedia         

Small and Medium-sized Businesses (SMB)             

Small and Medium-sized Enterprise (SME)             

Small Form-factor Pluggable (SFP)    Wikipedia         

Small Office/Home Office (SOHO)             

Smart Jack Type of Network Interface Device (NID) that includes circuitry to perform such functions as converting between framing formats on a digital circuit (T1), remote diagnostics, and regenerating a digital signal.            

SNMP agent Software that runs on a managed device such as a server, router, or switch.            

SNMP manager (aka: Network Management System (NMS)) Runs a network management application.            

socket The combination of the source and destination IP addresses and the source and destination port numbers.   Wikipedia         

socket pair the source and destination IP addresses and port numbers   Wikipedia         

Software-Defined Networking (SDN)  Began shortly after Sun Microsystems released Java in 1995.  Wikipedia         

source IP address Contains a 32-bit binary value that represents the source IP address of the packet.            

source MAC address Part of an Ethernet frame.             

source port The original protocol port that was used in in a data transfer.            

Spanning Tree Algorithm (STA) Determines the best paths to the root bridge in the broadcast domain.            

Spanning Tree PortFast BPDU Guard Enhancement (aka: BPDU Guard) Feature of the STP enhancement from Cisco that bypasses the spanning tree listening and learning process to speed up network convergence.    Cisco         

Spanning Tree Protocol (STP) 802.1D A legacy standard that runs all VLANs in a spanning tree instance. Developed to addresses the issue of routing loops. Ensures redundant physical links are loop-free. First standardized by IEEE as 802.1D in 1990. Original algorithm developed by Radia Perlman in 1985.  Cisco  Wikipedia  Cisco (Understanding and Configuring Spanning Tree Protocol)     

Spanning Tree Protocol Root Guard Enhancement (aka: root guard) Feature of the STP enhancement from Cisco that lets you administratively pick which bridge should be the root bridge. This works even if another switch is configured with a root priority of 0 and has a lower MAC address.   Cisco         

SPDY      Wikipedia       

SPF Tree Created by the SPF algorithm. Used to calculate the best path to each node.            

Split Horizon Prevents routing loops in distance vector routing protocols. Does not allow advertisement updates to be sent back down the interface it was learned. Can cause problems for frame relay.   Wikipedia         

SQL Slammer Worm Worm that exploited a bug in Microsoft SQL Server and Desktop Engine. Launcehed January 25, 2003.  Wikipedia         

SSH File Transfer Protocol (SFTP) (aka: Secure File Transfer Protocol) Secure file transfer protocol that uses SSH.   Wikipedia         

SSH forwarding Send another port over SSH.            

Standard ACL Permit or deny traffic only from source Layer 3 (IPv4) addresses. Should be placed close to the destination.            

Standby Active Virtual Gateway (SVG) Takes over for the AVG if it goes down. Elected by next highest priority to the AVG.            

Standby Virtual Forwarder (SVF) When more than 4 gateways are in a GLBP group then the rest will be SVF.            

Start of Authority (SOA) Type of DNS record.   Wikipedia         

startup configuration file Configuration file on network devices that is stored in NVRAM by default and accessed at startup.            

state table             

stateful protocol Protocol that keeps track of the state of the communication session.            

static NAT (aka: one-to-one) Statically configure the mapping of an inside local address (IP of internal server) to an inside global address (IP of server according to the internet).   Wikipedia  Juniper  Cisco     

static route Used to connect a specific network or to provide a Gateway of Last Resort. Have an AD of 1. They can be configured with the next-hop IP address (recursive static route), an exit interface (directly connected static route), or both an IP and exit interface (fully specified static route). Static routes are easy to implement in small networks. More secure and efficient than dynamic routes. Routes will always be the same and need to be changed manually as the topology grows/changes. To configure a static route use the syntax:

ip route [destination ip address] [subnet mask] [ exit interface | next-hop ip address ]


Statistical Time-Division Multiplexing (STDM) Uses a variable time slot where data can compete for a free slot. Developed to overcome TDM inefficiency.   Wikipedia         

Storage Area Network (SAN)    Wikipedia         

Straight Through Cable (T-568A or T-568B to itself) Most common networking cable. Connect from a host network interface controller (a computer or similar device) to a network switch, hub or router.    Wikipedia  LINFO       

Straight Tip (ST) (aka: stab and twist) Fiber-optic connector that uses a BNC attachment mechanism.   Wikipedia         

Stream Control Transmission Protocol (SCTP) Transport layer protocol similar to TCP and UDP. Standardized by IETF in RFC 4960 September 2007.  RFC 4960  Wikipedia       

stub network Router that is connected to a single router.   Wikipedia         

Stuxnet Worm that targets PLCs. Discovered in June 2010.  Wikipedia         

subnet ID Part of an IPv6 global unicast address. Organizations use this to identify subnets within a site.   Wikipedia         

subnet mask (netmask) Logicaly subdivides a network. Identifies what part of an address is the network portion and what is the host portion.   Wikipedia         

subnet zero The first subnet obtained after subnetting the network address.   Wikipedia  Cisco       

subnetwork (subnet) Logical subdivision of an IP network into two or more smaller networks.   Wikipedia         

Subscriber Connector (SC) (aka: square connector) Latched fiber-optic connector. Works with either SMF or MMF. Not as popular as ST connectors for LAN connections.   Wikipedia         

summary route Can help minimize the number of static routes in the routing table. IPv6 addresses can be summarized into a static route if: 1) the destination networks are contiguous and 2) the static routes all use the same exit interface or next-hop IPv6 address.   Wikipedia         

supernet route A routing table entry with a subnet mask less than the classful mask. (summary address)   Wikipedia         

switch Network device that typically operates at OSI Layer 2, Layer 3, or Layer 7. Provides centralized connectivity similar to hubs. Forwards data only to the device(s) that need to receive it based on the Layer 2 header. Filters MAC addresses and builds a MAC table. Switch has ports, router has interfaces. Can operate with or without VLANs. First Ethernet switch was introduced by Kalpana in 1990.  Wikipedia  Network Computing archive       

Switch Virtual Interface (SVI) logical interface associated with a VLAN. Can allow for remote switch administration. Used to assign an IP addresses to a Layer 2 switch where no physical interface for IP is available.            

Switched Port Analyzer (SPAN) Copies packets to another port on the switch to analyze.            

Switched Virtual Circuit (SVC) Established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION).   Netacad         

SYN/ACK Packet sent in response to a SYN packet.    Wikipedia         

Synchronous With clock rate.            

Synchronous Code-Division Multiple Access (S-CDMA)             

Synchronous Digital Hierarchy (SDH) European-based ETSI and ITU standard. Uses optical (fiber) to transmit data over a WAN connection.   Wikipedia         

Synchronous Optical Networking (SONET) ANSI standard. Uses optical (fiber) to transmit data over a WAN connection. (155.52Mbps)   Wikipedia         

System LifeCycle (SLC) The system lifecycle (SLC) should provide valuable guidance on the best practices that need to be followed throughout the network.            

Systems Network Architecture (SNA) IBM's proprietary networking architecture. Created September, 1974.  Wikipedia         

T1 (aka: T1 Line) Type of dedicated leased line. Composed of 24 64Kbps DS0 channels for a total bandwidth of 1.544Mbps. Originally used for telephony networks to carry one conversation in a single channel.  Wikipedia         

T3 Line    Wikipedia         

T568A   Wikipedia  Wikipedia       

T568B   Wikipedia  Wikipedia       

tag Helps with the frame election process.            

TCP Flag SYN, ACK, FIN, SYN/ACK            

TCP Segment Source Port, Destination Port, Sequence Number, Acknowledgement Number, Header length, Reserved, Control bits, Window, Checksum. Urgent, Options, Application Layer Data.   Wikipedia         


Telecommunications Industry Association (TIA)    Wikipedia         

Telelocator Alphanumeric Protocol (TAP)    Wikipedia         

Telenet Commercial packet switched network. Went into service in 1974.  Wikipedia         

Telnet Protocol used to provide remote CLI access devices such as servers and networking equipment. Does not encrypt transmission.   Wikipedia         

Test Anything Protocol (TAP)    Wikipedia         

TEST-NET Addresses Reserved for teaching and learning purposes   Wikipedia         

tethering Using a smartphone or tablet as a personal hotspot.            

three-tier hierarchical design Handles resources efficiently and scales easily.            

three-way handshake used by TCP   Wikipedia         

throughput The maximum amount of data that can be downloaded or uploaded at a time.   Wikipedia         

Tier 1 ISP Directly connected to the internet backbone. Primarily serves very large companies and Tier 2 ISPs            

Tier 2 ISP Primarily serves large companies and Tier 3 ISPs.            

Tier 3 ISP Primarily serves small/medium sized companies and homes.            

Time Division Multiple Access (TDMA)    Wikipedia         

Time Division Multiplexing (TDM) Splits the bandwidth of a single link into multiple time slots with a MUX.   Wikipedia         

tip and ring Red and green wires found in an RJ-11 wall jack, which carry voice, ringing voltage, and signaling information between an analog device (for example, a phone or a modem) and a telephone's wall jack.            

topology Layout of a network that describes where devices will go and how they will be connected. There are two kinds of topologies, logical and physical.   Wikipedia         

traffic classification Categorizes packets automatically based on parameters such as port or protocol.   Wikipedia         

transceiver Both a transmitter and a receiver. Similar to a repeater.   Wikipedia         

translation NAT64 allows IPv6 to communicate with IPv4            

Transport Control Protocol (TCP) (Layer 4 protocol.) Breaks data into segments for IP before they are sent. Full-duplex. Connection oriented. Has flow-control for each byte stream. TCP segments are marked with flags. Robert Kahn and Vitron Cerf began work on TCP in 1973.   Wikipedia  Wikipedia (List of Ports)       

Transport Control Protocol/Internet Protocol (TCP/IP) (aka: TCP/IP model, Department of Defense (DoD) model) Protocol suite. The IP suite is commonly known as TCP/IP because the foundational protocols in the suite are the Transmission Control Protocol (TCP) and the Internet Protocol (IP).

1 network interface
2 internet
3 transport
4 application

 Predates the OSI model, a more comprehensive reference framework for general networking systems. Replaced proprietary protocol suits such as AppleTalk and Novell's Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX).  Wikipedia         

Transport Layer (OSI Layer 4) Provides transport transfer of data between hosts. Responsible for services such as connection-oriented communication, reliability, flow control, and multiplexing. Layer 4 destination port is used to identify the upper layer protocol.   Wikipedia         

Transport Layer (OSI Layer 4) Conceptual layer in the internet protocol suite (TCP/IP) and the OSI model. Divides long communications into smaller data packages and handles error recognition and correction.   Wikipedia         

trap messages Solution to the main disadvantage of SNMP polling. Generated by SNMP agents and sent to the NMS to inform it of events without waiting for it to be polled.   Wikipedia         

Trivial File Transfer Protocol (TFTP) Used for connectionless active file transfer. Uses port 69.   Wikipedia         

trunk Single connection between two points such as a switch or branch office.   Wikipedia         

TrustSec Cisco borderless service. Secures the network based on polices in addition IP/MAC addresses. Different departments can determine who gets access to what while all being spelled out in plain English.   YouTube (Cisco - Fundamentals of TrustSec)  Cisco  YouTube (Cisco - TrustSec Takes the Complexity Out of Network Security)     

TUN/TAP Virtual network kernel devices.   Wikipedia         

Tunnel Mode (aka: IP-in-IP encryption) Secures the entire IP packet.    Netacad         

tunneling Transporting an IPv6 packet over an IPv4 network. IPv6 packet is encapsulated inside an IPv4 packet. (for networks that don't support dual stack)   Wikipedia         

two-tier hierarchical design Core and distribution layers implemented into a single layer.            

Type Length Value (TLV) The data portion of an EIGRP message that is encapsulated in a packet.   Wikipedia         

Type of Service (ToS) Part of an IPv4 packet header now represented by Differentiated Services Code Point (DSCP).   Wikipedia (ToS)  Wikipedia (IPv4 Header)       

ultimate route Routing table entry that has a next-hop IPv4 address or an exit interface. (Dynamic routes or local routes.)            

unequal-cost load balancing Distributes traffic over network interfaces even if they are different distances (metrics). Supported by EIGRP.            

unicast (One to One.) Sending a packet from one host to an individual host. Unicast ranges from but some address blocks are reserved for other special uses.   Wikipedia         

Uniform Resource Identifier (URI) Can be a URL or a URN.   Wikipedia         

Uniform Resource Locator (URL)    Wikipedia         

Unique Local Address (ULA) These addresses should not be routable in the global IPv6. Unique local addresses are in the range of FC00::/7 to FDFF::/7.  Defined in RFC 4193 October, 2005.  Wikipedia  RFC 4193       

Universal Mobile Telecommunications System (UMTS) Third generation mobile cellular system for networks based on the GSM standard.   Wikipedia         

Unshielded Twisted Pair (UTP) Cable found in many Ethernet networks and telephone systems. terminated with an RJ connector, Usually RJ-45 (8P8C) or RJ-11 (6P2C). TIA/EIA-568 is the standard for wire color codes to the pinouts for Ethernet cables.            

up/down status up/protocol down. Layer 2 issue: (keepalive, encapsulation, clocking)            

up/up status up/protocol up.             

User Datagram Protocol (UDP) Layer 4 connectionless protocol that has unreliable delivery. (Does not acknowledge if the data is received.) Uses less overhead than TCP.   Wikipedia         

V.35 Operates at Layer 1. Standard for modems.   Wikipedia  ITU       

V.92 ITU standard for 56 kb/s download and 48 kb/s upload modems. Introduced August 1999.  Wikipedia  ITU       

Variable Length Subnet Mask (VLSM) (classless subnetting) Helps avoid wasting IP addresses when subnetting a network.   Wikipedia         

Version Contains a 4-bit binary value identifying the IP packet version. For IPv4 packets, this field is always set to 0100.            

Very Small Aperture Terminal (VSAT) Creates a private WAN for connectivity to remote locations.   Wikipedia         

Virtual Circuit (VC) Two kinds of VCs: Switched Virtual Circuits (SVCs) and   Wikipedia         

Virtual Local-Area Network (VLAN) (aka: Virtual LAN) Similar to a regular LAN. Logical broadcast domain that can split physical LANs. Allows different security policies to be applied to different hosts.   Wikipedia  Cisco       

Virtual Router Redundancy Protocol (VRRP) IETF open standard that operates almost identically to HSRP from Cisco   Wikipedia         

Virtual Teletype (VTY) telnet   Wikipedia         

VLAN Double-Tagging Embeds a 802.1Q tag inside a frame allowing it to go to another VLAN. Because most switches only perform one level of decapsulation, the receiving switch only looks at the inner 802.1Q tag. (Outer tag would be the native VLAN).            

Voice over Asynchronous Transfer Mode (VoATM)    Wikipedia         

Voice over IP (VoIP) Requires no more than 150ms one-way delay, no more that 30ms of jitter, and no more thatn 1 percent packet loss.   Wikipedia         

voice traffic Maximum recommended one-way delay is 150ms for voice.            

VoIP Server (aka: IP PBX) An example VoIP server is Asterisk.            

VTY password Password used on a to secure a Telnet or SSH connection.   Cisco         

WAN Interface Card (WIC)    Wikipedia  Cisco       

WAN Switch      Cisco       

wavelength multiplexing Fame encoding technique used in fiber optic media.            

Wavelength-Division Multiplexing (WDM) Technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light.   Wikipedia         

Weave Communications layer for IoT devices. Developed by Google. Announced May 28, 2015.  Website  Ars Technica       

Web Security Appliance (WSA)             

Well-known Ports 0-1023            

whois Used to lookup domain name information.   Wikipedia  ICANN  RFC 3912     

Wi-Fi A WLAN technology. Uses the CSMA/CA media access process. Supposedly the name Wi-Fi is a play on words for Hi-Fi.   Wikipedia         

Wi-Fi Protected Access (WPA)    Wikipedia         

Wi-Fi Protected Access II (WPA2) (aka: 802.11i) Includes mandatory support for CCMP, an AES encryption. Replaced WPA. Required for all new devices to bear the Wi-Fi trademark starting March 13, 2006.  Wikipedia         

Wi-Fi Protected Access III (WPA3) Uses 192-bit encryption and individualized encryption for each user. Announced as a replacement to WPA2 by the Wi-Fi Alliance January 2018.  Wikipedia         

Wide Area Application Services (WAAS)             

Wide Area Network (WAN) Network that spans a large geographical area such as multiple buildings far apart on a campus. Uses mostly OSI layers 1 and 2. Connections are typically leased out by a service provider.   Wikipedia         

window size The amount of data that a source can transmit before an acknowledgement must be received. One of the fields in the TCP header that enables the management of lost data and flow control.   Wikipedia         


Wired Equivalent Privacy (WEP) (deprecated) Uses the RC4 stream cipher for confidentiality, and the CRC-32 checksum for integrity. Can be easily hacked. Introduced as part of the original 802.11 standard ratified in 1997. Superseded by Wi-Fi Protected Access (WPA) in 2003. Deprecated in 2004.  Wikipedia         

Wireless Access Point (WAP)    Wikipedia         

Wireless ad hoc network (WANET) A wireless LAN topology. When two or more devices are directly connected through a wireless network without extra infrastructure.   Wikipedia         

Wireless Control System (WCS)             

Wireless Internet Service Provider (WISP)             

Wireless LAN (WLAN) A LAN that uses radio waves to transmit data. Use the older CSMA/CA to decide when to gain access to the wireless media. Three main categories: IBSS, BSS, and ESS.   Wikipedia         

WLAN controller (WLC)             

WLAN coverage Recommended amount of coverage overlap in a 2.4GHz WLAN is 10-15% to prevent connections from dropping when roaming between coverage cells. Use nonoverlapping channels for adjacent cells.            

work area The outlets that connect to hosts through horizontal cabling. Two outlets should be provided at the wall plate; one for voice and one for data.   Wikipedia         

World Wide Web (WWW) (aka: the Web, port 80) HTTP/web services. Information space where documents and other web resources are identified by Uniform Resource Locators (URLs, such as, which may be interlinked by hypertext, and are accessible via the Internet. The resources of the WWW may be accessed by users via a software application called a web browser. Invented by English scientist Tim Berners-Lee while working for CERN by submitting Information Management: A Proposal on March 12, 1989.  Wikipedia         

Worldwide Interoperability for Microwave Access (WiMAX)    Wikipedia         

worm Malicious code that replicates itself. Responsible for some of the most devastating attacks on the internet. Primarily network based and can slow networks down. Can run on its own without user interaction. Tend to exploit software vulnerabilities.

Three major parts:
enabling vulnerability
propagation mechanism

 First used in John Brunner's 1975 novel, The Shockwave Rider. First appearance on the internet was the Morris worm released November 2, 1988.  Wikipedia  YouTube  Netacad     

X-Lite Free VoIP softphone   Website         

X.25 ITU-T standard protocol suite for packet switched wide area network (WAN) communication. Published in "The Orange Book" in 1976.  Wikipedia         

X.509 ITU-T standard for Public Key Infrastructure (PKI). Released July 3, 1988.  Wikipedia  ITU       

Yagi antenna Directional antenna used for long distances.   Wikipedia         

ZeroAccess Trojan horse that connects to a botnet.   Wikipedia         

Zeus Trojan horse. First identified July 2007.  Wikipedia         

zone file Text file containing DNS records for a domain name.