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).
The address is made up of the international data number (IDN), which consists of two fields: the 4 digit data network identification code (DNIC) and the (up to) 10 digit national terminal number (NTN).
The NTN identifies the exact network device (DTE, data terminal equipment) in the packet-switched network (PSN) and is often provided as an NUA.
The 14.0.0.0/8 block used to be reserved for X.121 use but was returned to IANA in 2008 to stave off IPv4 address exhaustion.