D

default gateway

A configuration item for the TCP/IP protocol that is the IP address of a directly reachable IP router. Configuring a default gateway creates a default route in the IP routing table.

See also Internet Protocol (IP).

See also IP address.

See also routing table.

See also Transmission Control Protocol/Internet Protocol (TCP/IP).

DHCP option

Address configuration parameters that a DHCP service assigns to clients. Most DHCP options are predefined, based on optional parameters defined in Request for Comments (RFC) 1542, although extended options can be added by vendors or users.

See also Dynamic Host Configuration Protocol (DHCP).

DHCP/BOOTP Relay Agent

The agent program or component responsible for relaying Dynamic Host Configuration Protocol (DHCP) and bootstrap protocol (BOOTP) broadcast messages between a DHCP server and a client across an Internet Protocol (IP) router. A DHCP relay agent supports DHCP/BOOTP message relay as defined in RFCs 1541 and 2131. The DHCP Relay Agent routing protocol component is managed using the Routing and Remote Access snap-in.

See also bootstrap protocol (BOOTP).

See also Dynamic Host Configuration Protocol (DHCP).

dial-up connection

The connection to your network if you use a device that uses the telephone network. This includes modems with a standard telephone line, ISDN cards with highspeed ISDN lines, or X.25 networks.

If you are a typical user, you might have one or two dial-up connections, for example, to the Internet and to your corporate network. In a more complex server situation, multiple network modem connections might be used to implement advanced routing.

See also Integrated Services Digital Network (ISDN).

dictionary attack

A method of guessing a user's password or PIN by trying every word in the dictionary until successful.

digital signature

A means for originators of a message, file, or other digitally encoded information to bind their identity to the information. The process of digitally signing information entails transforming the information, as well as some secret information held by the sender, into a tag called a signature. Digital signatures are used in public key environments, and they provide nonrepudiation and integrity services.

digital subscriber line (DSL)

A special communication line that uses modulation technology to maximize the amount of data that can be sent over a local-loop, copper-wire connection to a central phone office. DSL is used for connections from telephone switching stations to a subscriber rather than between switching stations.

distributed DHCP

An implementation in which DHCP distributes IP addresses across a site boundary.

See also Dynamic Host Configuration Protocol (DHCP).

See also IP address.

Distributed File System (DFS)

A service that allows system administrators to organize distributed network shares into a logical namespace, enabling users to access files without specifying their physical location and providing load sharing across network shares.

domain

In Active Directory, a collection of computer, user, and group objects defined by the administrator. These objects share a common directory database, security policies, and security relationships with other domains.

In DNS, any tree or subtree within the DNS namespace. Although the names for DNS domains often correspond to Active Directory domains, DNS domains should not be confused with Active Directory domains.

See also Active Directory.

See also Domain Name System (DNS).

domain controller

In an Active Directory forest, a server that contains a writable copy of the Active Directory database, participates in Active Directory replication, and controls access to network resources. Administrators can manage user accounts, network access, shared resources, site topology, and other directory objects from any domain controller in the forest.

See also Active Directory.

See also authentication.

See also forest.

domain name

The name given by an administrator to a collection of networked computers that share a common directory. Part of the DNS naming structure, domain names consist of a sequence of name labels separated by periods.

See also domain.

See also Domain Name System (DNS).

Domain Name System (DNS)

A hierarchical, distributed database that contains mappings of DNS domain names to various types of data, such as IP addresses. DNS enables the location of computers and services by user-friendly names, and it also enables the discovery of other information stored in the database.

See also domain name.

See also IP address.

See also Transmission Control Protocol/Internet Protocol (TCP/IP).

Dynamic Host Configuration Protocol (DHCP)

A TCP/IP service protocol that offers dynamic leased configuration of host IP addresses and distributes other configuration parameters to eligible network clients. DHCP provides safe, reliable, and simple TCP/IP network configuration, prevents address conflicts, and helps conserve the use of client IP addresses on the network.

DHCP uses a client/server model where the DHCP server maintains centralized management of IP addresses that are used on the network. DHCP-supporting clients can then request and obtain lease of an IP address from a DHCP server as part of their network boot process.

See also IP address.

See also lease.

See also Transmission Control Protocol/Internet Protocol (TCP/IP).

dynamic update

An update to the Domain Name System (DNS) standard that permits DNS clients to dynamically register and update their resource records in zones.

See also Domain Name System (DNS).