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X.121

A CCITT Recommendation that provides a method for the international numbering of X.25 packet-switching data networks.

X.21

(1) See Recommendation X.21.

(2) See specification X.21.

X.21bis

In data communications, an interim specification of the CCITT that defines the connection of data terminal equipment (DTE) to an X.21 (public data) network using V-series interchange circuits such as those defined by CCITT V.24 and CCITT V.35.

X.25

A CCITT standard that defines an interface to packet switched communications services. A document, CCITT Recommendation X.25, outlines standards for the connection of processing equipment to a packet-switching data network. Also known as ISO 8208.

X.25 protocol

The connection-mode network service that the CCITT specifies in Recommendation X.25.

X.25 protocol ID

In OSI, a technique that OSI Communications Subsystem uses to pass an NSAP address when using the 1980 level of the X.25 protocol.

X.25 subnetwork

In OSI, a subnetwork that conforms to the X.25 protocols.

X.400

A CCITT Recommendation for international electronic mail (messages).

X.500

The directory services standard of ITU, ISO, and IEC.

X.75

In data communications, a specification that defines ways of connecting two X.25 networks.

XID

See exchange station ID.

XML

See Extensible Markup Language.

XSM

See cross-site mirroring.