(Historical) The CCITT recommendation for a numbering plan for public data network

International numbering plan for PDNs.

The 121st CCITT recommendation in the X series, defining a convention for the network user address (NUA).

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

ITU-T standard describing an addressing scheme used in X.25 networks. Sometimes called the X.25 address.

The addressing format used by X.25 base networks.

The ITU-T standard for the addressing plan used with X.25 PDNs.

X.121 is the ITU-T address format of the X.25 protocol suite used as part of call setup to establish a switched virtual circuit between Public Data Networks (PDNs), connecting two network user addresses (NUAs). It consists of a maximum of fourteen binary-coded decimal digits and is sent over the Packet Layer Protocol (PLP) after the packet type identifier (PTI).