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Glossary: CCNA & General Networking Terms 


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Defense Advanced Research Projects Agency. See: Advanced Research Projects Agency
Data Encryption Key (DEK)
Used for the encryption of message text and for the computation of message integrity checks (signatures). See also: encryption. [Source: RFC1392]
Data Encryption Standard (DES)
A popular, standard encryption scheme. See also: encryption, Pretty Good Privacy, RSA. [Source: RFC1983]
A self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer and the transporting network. See also: frame, packet. [Source: J. Postel]
Data Link Layer
The OSI layer that is responsible for data transfer across a single physical connection, or series of bridged connections, between two Network entities. [Source: RFC1208]
See: Defense Information Systems Agency
DCE (1)
Data Circuit-terminating Equipment
DCE (2)
See: Distributed Computing Environment
See: Defense Data Network
See: Defense Data Network Network Information Center
A proprietary network protocol designed by Digital Equipment Corporation. The functionality of each Phase of the implementation, such as Phase IV and Phase V, is different. [Source: RFC1392]
default route
A routing table entry which is used to direct packets addressed to networks not explicitly listed in the routing table. [Source: MALAMUD]
Defense Advanced Research Projects Agency (DARPA)
See: Advanced Research Projects Agency.
Defense Data Network (DDN)
A global communications network serving the US Department of Defense composed of MILNET, other portions of the Internet, and classified networks which are not part of the Internet. The DDN is used to connect military installations and is managed by the Defense Information Systems Agency. See also: Defense Information Systems Agency. [Source: RFC1392]
Defense Data Network Network Information Center (DDN NIC)
Often called "The NIC", the DDN NIC's primary responsibility is the assignment of Internet network addresses and Autonomous System numbers, the administration of the root domain, and providing information and support services to the DDN. It is also a primary repository for RFCs. See also: Autonomous System, network address, Internet Registry, Network Information Center, Request For Comments. [Source: RFC1392]
Defense Information Systems Agency (DISA)
Formerly called the Defense Communications Agency (DCA), this is the government agency responsible for managing the DDN portion of the Internet, including the MILNET. Currently, DISA administers the DDN, and supports the user assistance services of the DDN NIC. See also: Defense Data Network. [Source: RFC1392]
See: Data Encryption Key
See: Data Encryption Standard
See: Dynamic Host Configuration Protocol
A temporary, as opposed to dedicated, connection between machines established over a standard phone line. [Source: RFC1392]
Directory Access Protocol
X.500 protocol used for communication between a Directory User Agent and a Directory System Agent. [Source: MALAMUD]
Directory System Agent (DSA)
The software that provides the X.500 Directory Service for a portion of the directory information base. Generally, each DSA is responsible for the directory information for a single organization or organizational unit. [Source: RFC1208]
Directory User Agent (DUA)
The software that accesses the X.500 Directory Service on behalf of the directory user. The directory user may be a person or another software element. [Source: RFC1208]
See: Defense Information Systems Agency
Converting a binary program into human-readable machine language code. See also PAD.
Distributed Computing Environment (DCE)
An architecture of standard programming interfaces, conventions, and server functionalities (e.g., naming, distributed file system, remote procedure call) for distributing applications transparently across networks of heterogeneous computers. Promoted and controlled by the Open Software Foundation (OSF), a consortium led by Digital, IBM and Hewlett Packard. [Source: RFC1208]
distributed database
A collection of several different data repositories that looks like a single database to the user. A prime example in the Internet is the Domain Name System. [Source: RFC1392]
DIX Ethernet
See: Ethernet
See: Domain Name System
Document Type Definition
Definition of a language built on eXtensible Markup Language (XML) or Standardised General Markup Language (SGML).
"Domain" is a heavily overused term in the Internet. It can be used in the Administrative Domain context, or the Domain Name context. See also: Administrative Domain, Domain Name System. [Source: RFC1392]
Domain Name System (DNS)
The DNS is a general purpose distributed, replicated, data query service. The principal use is the lookup of host IP addresses based on host names. The style of host names now used in the Internet is called "domain name", because they are the style of names used to look up anything in the DNS. Some important domains are: .COM (commercial), .EDU (educational), .NET (network operations), .GOV (U.S. government), and .MIL (U.S. military). Most countries also have a domain. The country domain names are based on ISO 3166. For example, .US (United States), .UK (United Kingdom), .AU (Australia). It is defined in STD 13, RFC 1034 and RFC 1035. See also: Fully Qualified Domain Name, Mail Exchange Record. [Source: RFC1983]
dot address (dotted decimal notation)
Dot address refers to the common notation for IP addresses of the form A.B.C.D; where each letter represents, in decimal, one byte of a four byte IP address. See also: IP address. [Source: FYI4]
Data Protection Act.
A framing specification for T-1 synchronous lines. See also: T1 [Source: RFC1392]
A framing specification for T-3 synchronous lines. See also: T3 [Source: RFC1392]
See: Directory System Agent
Data Service Unit, a physical cable interface on a network link.
See: Document Type Definition
Data Terminal Equipment
See: Directory User Agent
dynamic adaptive routing
Automatic rerouting of traffic based on a sensing and analysis of current actual network conditions. NOTE: this does not include cases of routing decisions taken on predefined information. [Source: J. Postel]
Dial-up User List, which has entries for ranges of IP addresses used by Internet Service Providers for modem users.
Dense Wave Division Multiplexing, a technique for passing multiple data streams along an optical fibre cable using different wavelengths as carriers.
Dynamic Host Configuration Protocol (DHCP)
A framework for passing configuration information to hosts on a TCP/IP network, using an options field; a development from the BOOTP Bootstrap Protocol. Described in RFC 1533 and RFC 1534 and updated in RFC 2132. See also: BOOTP.


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