[X]

X. – A series of CCITT recommendations for data transmission over public data networks.

X.21 – A CCITT recommendation which specifies a general purpose interface between data circuit-terminating equipment for synchronous operation on public data networks.

X.25 – A CCITT recommendation which specifies an interface between data terminal equipment for terminals operating in the packet mode on public data network.

X.75 – A CCITT recommendation which specifies the protocol used to communicate between packet switched networks.

X.121 – A CCITT recommendation which specifies an international numbering plan for addressing data networks.

X.400 – A CCITT recommendation which specifies an international numbering plan for public message handling systems.