X

X.121: A CCITT Recommendation that provides a method for the international numbering of X.25 packet-switching data networks.
X.21: See Recommendation X.21.; 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.
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.; See exchange identification.
XML: See Extensible Markup Language.
XSM: See cross-site mirroring.