229 results

D Identifies a dynamically learned network from another router using the EIGRP routing protocol in a routing table.   Wikipedia         

daemon Program that runs as a background process.   Wikipedia         

Darwin Open source OS based on BSD and made by Apple. OS X and iOS get their core components from Darwin. First released November 15, 2000.  Wikipedia  Website       

data (information) The smallest form is a bit but can be represented by byte, kilobyte (KB), megabyte (MB), gigabyte (GB), terabyte (TB), petabyte (PB), exabyte (EB), zettabyte (ZB), yottabyte (YB).   Wikipedia         

Data Access Layer (DAL)    Wikipedia         

data center Typically a data storage and processing facility run by an in-house IT department or leased offsite.   Wikipedia  Netacad       

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 Deduplication Required steps: Ensure shared files and folders are not stored on the system volume. Must be NTFS; does not support ReFS.            

Data Encapsulation    Wikipedia  YouTube       

Data Encryption Standard (DES) Symmetric encryption algorithm, usually encrypted with a block cipher. The key is 64-bits long, but only 56 bits are used for encryption. Can be used by IPsec to encrypt data. Published as FIPS standard FIPS PUB 46 January 15, 1977.  Wikipedia  Netacad       

data integrity (checksum) Maintenance of, and the assurance of the accuracy and consistency of data over its entire life-cycle, and is a critical aspect to the design, implementation and usage of any system which stores, processes, or retrieves data. The term is broad in scope and may have widely different meanings depending on the specific context – even under the same general umbrella of computing.   Wikipedia         

Data Manipulation Commands (DML) Allow you to add, modify, and delete rows from tables. Basic commands are; SELECT, INSERT, UPDATE, DELETE, COMMIT, and, ROLLBACK.             

Data Recovery Agent (DRA)             

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

data structure A way of organizing data.   Wikipedia         

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

data type A classification identifying the type of data. (i.e.    Wikipedia         

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

database A collection of data.   Wikipedia         

Database Description (DBD) One of the OSPF protocol messages.            

Database Handle (DBH) ($dbh)            

Database management systems (DBMS) Software that interacts with end users, applications, and the database itself to capture and analyze data. The order of rows and columns is not important.   Wikipedia         

Database Source Name (DSN)    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)

date Command that displays or sets the date and time in many operating systems. Examples:

Linux: sudo date -s "Jul 5 08:10"
Windows: date (Output is the same as Get-Date in PowerShell)
   Wikipedia  TechNet       

DB-9 Standard end used on serial cables.   Wikipedia         

DC-to-DC converter Converts DC from one voltage level to another.   Wikipedia         

dd Vi command used to delete the current line.            

ddate Shows the Discordian date.     About.com       

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

Deadlock waiting for a process that never finishes.   Wikipedia         

Debian A Linux distribution.   Wikipedia  Website       

Debian Almquist Shell (Dash) Light weight version of Bash.   ArchWiki         

Debian Package Management System Runs on dpkg   Wikipedia  Wikipedia       

debug ip bgp             

debug ip nat s=source address, a.b.c.d--->w.x.y.z=source address--->translated address, d=destination ip address, [xxxx]=ip identification number            

Decent Security    Website         

declare Bash builtin that sets attributes for shell variables. Use - to set an attribute and + to remove it.            

DEF CON (aka: DEFCON) One of the world's largest security/hacker conventions. First held June 9, 1993.  Website  Wikipedia       

DEF CON 26  Caesar's Palace & Flamingo, Las Vegas August 9-12, 2018.  Website  Wikipedia       

DEF CON 27  August 8-11, 2019 at Paris, Ballys, & Planet Hollywood, Las Vegas.  Website         

DEF CON 28 SAFE MODE  August 6-9, 2020 online.  Website         

DEF CON 29  Aug. 5-8, 2021  Website         

DEF CON 30  Aug. 11-14, 2022  Website         

DEF CON China 2  On Hold - Dates TBD. 751D Park in Beijing, China.  Website         

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.             

degrees HTML: & deg;            

del Command that deletes one or more files. Use rm in UNIX.   Wikipedia         

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

Dell Private computer company. Founded February 1, 1984.  Website  Wikipedia  Support     

Dell EMC (formerly EMC Corporation) Founded in 1979.  Website  Wikipedia       

Dell PowerEdge (PE) Dell's server product line.   Website  Wikipedia       

Dell Remote Access Controller (DRAC) Out-of-band management platform on certain Dell servers. The platform may be provided on a separate expansion card, or integrated into the main board; when integrated, the platform is referred to as iDRAC.   Wikipedia         

Dell Technologies Parent company of Dell created after the merger of Dell Inc. and EMC Corporation. Founded September 7, 2016.  Website  Wikipedia       

delta Δ Change.   Wikipedia         

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

demidecode      die.net       

Demilitarized Zone (DMZ) Section of a network containing servers that must connect to an external network, such as the internet.   Wikipedia  Netacad       


Denial of Service (DoS) A DoS attack works by continuously sending packets of unexpected size or unexpected data. This results in interruption of service to users.   Wikipedia  Netacad       

Dennis Ritchie  Developed the C programming language with help from Ken Thompson in 1973.  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         

dependent Value of an attribute that is determined by another attribute.            

Deployment Image Servicing and Management (DISM)    MSDN         

DESCHALL First group to publicly crack the Data Encryption Standard (DES).  Announced June 18, 1997.  Wikipedia  This Day in Tech History       

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

Designated Router (DR) uses multicast address for updates.            

Desktop Management Interface (DMI)    Wikipedia         

Desktop Services Store (.DS_Store) A hidden file created by Finder in OS X.   Wikipedia         

desktop.ini    Wikipedia         

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             

detective controls (during the event) Identify and characterize an incident in progress e.g. by sounding the intruder alarm and alerting the security guards or police.   Wikipedia         

determination The role of a key. (A determines B)            

Development and Operations (DevOps)    Wikipedia         

DFS Namespace    Website  Wikipedia       

DFS Replication    Website  Wikipedia       

DHCP snooping    Wikipedia         

Diagnostics and Recovery Toolset (DaRT) Helps diagnose an offline copy of Windows   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         

Diameter protocol    Wikipedia         

die Part of an integrated circuit.   Wikipedia         

dielectric Insulating material used in capacitors.   Wikipedia         

Difference of Cubes Factoring: A3 - B3 = (A - B)(A2 + AB + B2)   Khan Academy         

difference of squares x2+y2 = (a+b)(a-b)            

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)     

Diffie–Hellman key Exchange (DHE) (aka: Diffie-Hellman (DH)) Key negotiation and agreement protocol used in public key cryptography. Asymmetric algorithm used to create shared keys. Does not provide encryption but is used to establish the secret key between two parties. First published by Whitfield Diffie and Martin Hellman in 1976.

DH1 (Legacy) 768 bit key.
DH2 (Legacy) 1024 bit key
DH5 (Legacy) 1536 bit key
DH14 2048 bit key
DH15 3072 bit key
DH16 4096 bit key
DH19 supports Elliptical Curve Cryptography
DH20 supports Elliptical Curve Cryptography
DH24 supports Elliptical Curve Cryptography
  Wikipedia  Netacad  Netacad  YouTube (Art of the Problem)   

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         

digest Fixed length output "thumbprint" for the data being hashed.   Wikipedia         

Digital Attack Map Top daily DDoS attacks worldwide.   Website         

digital certificate (aka: public key certificate) Includes the public key, digital signature, and verification from a third party.   Wikipedia  YouTube (itfreetraining)       

digital certificates Exchanged to authenticate peers            

Digital Equipment Corporation (DEC) Developed the PDP line of computers. Created the first firewall in 1988. Later acquired by Compaq.  Wikipedia         

Digital Linear Tape (DLT) (formerly: CompacTape) Magnetic tape data storage technology. Maximum capacity of 800GB or 160GB in the value line. Developed by Digital Equipment Corporation (DEC) in 1984.  Wikipedia         

Digital Rights Management (DRM)             

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         

Disable-ADAccount PowerShell cmdlet used to disable an Active Directory account.   TechNet         

dism.exe /online /cleanup-image /restorehealth Command that will attempt to repair your corrupted file from a trusted online source (Windows Update). Requires administrative privileges.   Tom's Hardware         

Double Data Rate Synchronous DRAM (DDR SDRAM)    Wikipedia         

Drupal Free, open-source CMS framework written in PHP. First released May 18, 2000.  Website  Wikipedia       

Dynamic Disks    MSDN  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 signature (aka: hash) Mathematical method used to check the authenticity and integrity of a message, digital document, or software. Gives a recipient reason to believe that the data was created by a known sender (authentication), that the sender cannot deny having sent the data (non-repudiation), and that the data was not altered in transit (integrity). Used when connecting to a secure website. Can be used as an alternative to HMAC.   Wikipedia         

Digital Signature Algorithm (DSA) (authentication) Federal Information Processing Standard (FIPS) for digital signatures, based on the mathematical concept of modular exponentiation and the discrete logarithm problem. DSA is a variant of the Schnorr and ElGamal signature schemes. DSA signature generation is faster than DSA signature verification. Attributed to David W. Kravitz. Filed under U.S. Patent 5,231,668 July 26, 1991. Adopted by the U.S. government in 1993 with FIPS 186.  Wikipedia         

Digital Signature Standard (DSS)             

Digital Subscriber Line (DSL) A type of broadband internet connection media that uses phone lines. (WAN)            

Digital Subscriber Line Access Multiplexer (DSLAM)    Wikipedia         

Digital Versatile Disc (DVD) Typically 4.7 GB to 8.5 GB.

DVD 4.7 GB
MiniDVD 1.46 GB
MiniDVD DL 2.66 GB

Digital Video Broadcasting (DVB)    Wikipedia         

Digital Video Broadcasting-Common Interface (DVB-CI)    Wikipedia         

Digital Visual Interface (DVI)

DVI-I (integrated, combines digital and analog in the same connector; digital may be single or dual link)
DVI-D (digital only, single link or dual link)
DVI-A (analog only)

 Developed April 1999.  Wikipedia         

Digital, Intel, Xerox (DIX)             

digital-to-analog converter (DAC)    Wikipedia         

Dijkstra's algorithm Used in OSPF.            

DIN connector    Wikipedia         

dir Command that displays a list of files and subdirectories in a directory. Use ls in UNIX-based systems or Windows through PowerShell.   Wikipedia  TechNet       

Direct Current (DC) Current goes only one direction. Favored by Thomas Edison.  Wikipedia         

Direct Media Interface (DMI)    Wikipedia         

Direct Memory Access (DMA)    Wikipedia         

Direct Memory Access (DMA) attack             

Direct Memory Interface (DMI)    Website  Wikipedia       

Direct X Diagnostic Tool (dxdiag) Program that displays information about DirectX. (version,            

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         

DirectAccess Automatically establishes a secure bi-directional IPSec connection. Introduced with Windows 7 and Windows Server 2008 R2.  Website  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.            

directory Another name for a folder. Holds other files or dirctories. Represented by eitehr / (Linux) or \ (Windows).            

Directory Services Restore Mode (DSRM) Used in a domain controller to take Active Directory offline.   Wikipedia         

Directory System Agent (DSA) The directory service component that runs as Ntdsa.dll on each domain controller, providing the interfaces through which services and processes gain access to the directory database.   TechNet         

Dirty Change on Write (Dirty COW) (aka: CVE-2016-5195) Linux kernel vulnerability.   Linux.com  Red Hat       

disabled port Shutdown port.            

disk spinning platters (Hard drives)            

disk array    Wikipedia         

Disk Operating System (DOS)    Wikipedia         

DISKCOMP Compares the contents of two floppy disks.            

DISKCOPY Copies the contents of one floppy disk to another.            

DISKPART DOS command used to manage disk partitions.    TechNet         

DisplayPort (DP) Digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association (VESA). Version, 1.0 approved by VESA on May 3, 2006.  Wikipedia         

Distance Vector Routing Protocol Router only know what is next to them. Does not have an actual map of the whole network.            

Distinguished Name (DN) (aka: LDAP Distinguished Name (DN)) Unique database name for every Active Directory (AD) object. Made up of Relative Distinguished Name (RDN).   Wikipedia  YouTube (Microsoft MCSA/MCSE Learning Channel)       

Distributed Component Object Model (DCOM)    Website  Wikipedia       

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

Distributed Denial of Service (DDoS) A DDoS attack is similar to a DoS attack but originates from multiple sources.   Wikipedia  Network       

Distributed File System (DFS)      Wikipedia       

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

Distribution System (DS) The bridge between 2 BSSs            

Django High-level web development framework written in Python. First released July 21, 2005.  Website  Wikipedia       


DNS caching How long a DNS server will hold a record before checking for updates. Usually determined by TTL.            

DNS poisoning (DNS spoofing) Attack that introduces erroneous or malicious entries into a DNS server’s zone file or a server’s hostname-to-IP address cache, intending to misdirect DNS resolution requests.   Wikipedia         

DNS query Request for name resolution.   Wikipedia         

DNS Server Maps name-to-IP addresses for internal hosts. Forwards name resolution requests between servers. Stores resource records used to resolve names which contain the name, address, and type of record. Some types are: A (an end device address), NS (an authoritative name server), CNAME, MX (Mail Exchange).            

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

doc .doc Proprietary filename extension for a Microsoft Word document.   Wikipedia          

Docker Allows you to package an application with all of its dependencies into a standardized unit for software development.   Website         

DockerCon 2018  June 12-24 in San Francisco, California.  Website         

Document Object Model (DOM)    Wikipedia         

docx A Microsoft Word document.   Wikipedia         

dollar sign underscore $_ Object being passed through the pipeline.            

domain Collection of user accounts, computers, printers, and other objects managed in a database.   Wikipedia         

domain Used to organize and describe an attribute's set of possible values in data modeling.            

Domain Component (DC)    TechNet  YouTube (Microsoft MCSA/MCSE Learning Channel)       

Domain Controller (DC) A Windows server used to control user permissions. Sometimes referred to as an Active Directory Server.   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 google.com 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.            

Doom First-person shooter. First released December 10, 1993 to MS-DOS and Mac OS.  Wikipedia         

doping Adding impurity atoms to a semiconducting material. Can be p-type or n-type.   Wikipedia         

dormant cyber pathogen What Michael Ramos, the San Bernardino County district attorney, claims was on a terrorist's iPhone. Used in a hearing for the San Bernardino iPhone case on March 22, 2016.  Ars Technica         

DOSKEY Edits command lines, recalls Windows commands, and creates macros.            

Double Data Rate version three Synchronous DRAM (DDR3 SDRAM)  Release in 2007.  Wikipedia         

Double Data Rate version two Synchronous DRAM (DDR2 SDRAM)  Release in 2003.  Wikipedia         

double-precision (IEEE 754)    Wikipedia         

dpkg (Debian package) A command that can be used to install and remove packages in a Debian based OS. It can also be used to view the currently installed packages.   Wikipedia         

dpkg-reconfigure tzdata Reconfigure the date and timezone on a Debian-based Linux distribution. Example:

sudo dpkg-reconfigure tzdata
   Raspberry Pi Forums         

Dreamcast Video game console made by Sega. Released in Japan November 27, 1998. Released in North America September 9, 1999.  Wikipedia         

driver Allows the OS to communicate with specific hardware.            

driverless cars  Approved by the California Department of Motor Vehicles (DMV) February 26, 2018. Regulations in effect as of April 2, 2018.  Ars Technica  DMV       

DRIVER_IRQL_NOT_LESS_OR_EQUAL (d1) An attempt was made to access a pageable (or completely invalid) address at an interrupt request level (IRQL) that is too high. This is usually caused by drivers using improper addresses.   Windows Debugging Tools         

Droid Razr  First released November 11, 2011.  Wikipedia         

Dropbox File hosting service. Release June 2007.  Website  Wikipedia       

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

Dual In-line Memory Module (DIMM) Type of memory module that typically comes with 168, 184, 240, or 288 pins. Larger than a SIMM slot.

100-pin DIMM printer SDRAM
168-pin DIMM SDR SDRAM (less frequently for FPM/EDO DRAM in workstations/servers, may be 3.3 or 5 V)
200-pin DIMM FPM/EDO DRAM in some Sun workstations and servers
278-pin DIMM HP high density SDRAM

Dual In-Line Package (DIP)    Wikipedia         

dual stack Allows IPv4 and IPv6 to coexist on the same network.            

Dual-Tone Multi-Frequency signaling (DTMF)    Wikipedia         

duck typing    Wikipedia         

dumb terminal             

Duo Video chat app from Google. Announced May 18, 2016. Available for Android and iOS August 16, 2016.  Website  Wikipedia  CNET     

duplex settings half duplex and full duplex             

Duplicate Address Detection (DAD) Checks for duplicate IPv6 unicast addresses.            

DVD Double Layer (DVD DL) 8.55 GB

DVD-R DL 8,543,666,176 Bytes
DVD+R DL 8,547,991,552 Bytes

DVD Double Sided (DVD DS) 9.4 GB

DVD-R DS, DVD-RW DS 9,414,639,616 Bytes
DVD+R DS, DVD+RW DS 9,400,745,984 Bytes

DVD Double Sided, Double-Layer (DVD DS, DL) (aka: dual-layer) 17.09 GB

DVD-R DS, DL 17,087,332,352 Bytes
DVD+R DS, DL 17,095,983,104 Bytes

DVD+R Single Sided, Single Layer DVD recordable (DVD+R) and DVD rewritable (DVD+RW) discs have a 4.7 GB capacity.

DVD+R, DVD+RW 4,700,372,992 Bytes
 Developed by Philips and Sony along with several other companies in the DVD+RW Alliance mid 2002.  Wikipedia         

DVD-R Single Sided, Single Layer DVD recordable (DVD-R) and DVD rewritable (DVD-RW) discs have a 4.7 GB capacity.

DVD-R, DVD-RW 4,707,319,808 Bytes
 Developed by Pioneer in 1997.  Wikipedia         

dvorak An alternative English keyboard layout to QWERTY that is designed to be more efficient.   Wikipedia         

Dynamic Access Control (DAC)             

Dynamic ACL    Cisco         

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 Memory Feature in Microsoft Hyper-V that let virtual machines use more physical memory than assigned. Introduced in Windows Server 2008 R2 Service Pack 1 (SP1).  TechNet         

Dynamic Multipoint VPN (DMVPN)    Wikipedia  Cisco (DMVPN)  Cisco (Configure DMVPN using GRE over IPSec between Multiple Routers)  YouTube (Keith Barker)   

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 programming language Type of high-level programming language.   Wikipedia         

Dynamic Random Access Memory (DRAM) Type of RAM that stores each bit of data in a separate capacitor within an integrated circuit.   Wikipedia         

dynamic route             

dynamic routing protocol (EIGRP, RIPv2            

Dynamic Trunking Protocol (DTP) Cisco proprietary.   Wikipedia