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and the network. (I) (2) In TCP/IP, the basic unit of
information passed across the Internet environment. A
datagram contains a source and destination address
along with the data. An Internet Protocol (IP) datagram
consists of an IP header followed by the transport layer
data. (3) See also
packet
and
segment
.
Datagram Delivery Protocol (DDP). In AppleTalk
networks, a protocol that provides network connectivity
by means of connectionless socket-to-socket delivery
service on the internet layer.
DCE ready. In the EIA 232 standard, a signal that
indicates to the data terminal equipment (DTE) that the
local data circuit-terminating equipment (DCE) is
connected to the communication channel and is ready
to send data. Synonymous with
data set ready (DSR)
.
DECnet. A network architecture that defines the
operation of a family of software modules, databases,
and hardware components typically used to tie Digital
Equipment Corporation systems together for resource
sharing, distributed computation, or remote system
configuration. DECnet network implementations follow
the Digital Network Architecture (DNA) model.
default. Pertaining to an attribute, condition, value, or
option that is assumed when none is explicitly
specified. (I)
dependent LU requester (DLUR). An APPN end node
or an APPN network node that owns dependent LUs,
but requests that a dependent LU server provide the
SSCP services for those dependent LUs.
designated router. A router that informs end nodes of
the existence and identity of other routers. The selection
of the designated router is based upon the router with
the highest priority. When several routers share the
highest priority, the router with the highest station
address is selected.
destination node. The node to which a request or
data is sent.
destination port. The 8-port asynchronous adapter
that serves as a connection point with a serial service.
destination service access point (DSAP). In SNA
and TCP/IP, a logical address that allows a system to
route data from a remote device to the appropriate
communications support. Contrast with
source service
access point (SSAP)
.
device. A mechanical, electrical, or electronic
contrivance with a specific purpose.
digital. (1) Pertaining to data that consist of digits. (T)
(2) Pertaining to data in the form of digits. (A) (3)
Contrast with
analog
.
Digital Network Architecture (DNA). The model for
all DECnet hardware and software implementations.
direct memory access (DMA). The system facility
that allows a device on the Micro Channel bus to get
direct access to the system or bus memory without the
intervention of the system processor.
directory. A table of identifiers and references to the
corresponding items of data. (I) (A)
directory service (DS). An application service element
that translates the symbolic names used by application
processes into the complete network addresses used in
an OSI environment. (T)
directory services (DS). A control point component of
an APPN node that maintains knowledge of the location
of network resources.
disable. To make nonfunctional.
disabled. (1) Pertaining to a state of a processing unit
that prevents the occurrence of certain types of
interruptions. (2) Pertaining to the state in which a
transmission control unit or audio response unit cannot
accept incoming calls on a line.
domain. (1) That part of a computer network in which
the data processing resources are under common
control. (T) (2) In Open Systems Interconnection (OSI),
a part of a distributed system or a set of managed
objects to which a common policy applies. (3) See
Administrative Domain
and
domain name
.
domain name. In the Internet suite of protocols, a
name of a host system. A domain name consists of a
sequence of subnames separated by a delimiter
character. For example, if the fully qualified domain
name (FQDN) of a host system is ralvm7.vnet.ibm.com,
each of the following is a domain name:
v ralvm7.vnet.ibm.com
v vnet.ibm.com
v ibm.com
domain name server. In the Internet suite of
protocols, a server program that supplies
name-to-address translation by mapping domain names
to IP addresses. Synonymous with
name server
.
Domain Name System (DNS). In the Internet suite of
protocols, the distributed database system used to map
domain names to IP addresses.
dotted decimal notation. The syntactical
representation for a 32-bit integer that consists of four
8-bit numbers written in base 10 with periods (dots)
separating them. It is used to represent IP addresses.
dump. (1) Data that has been dumped. (T) (2) To
copy the contents of all or part of virtual storage for the
purpose of collecting error information.
dynamic reconfiguration (DR). The process of
changing the network configuration (peripheral PUs and
Glossary 697