X.121 is the

ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...

address format of the

X.25 X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts ...

protocol suite The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family. Some of these terms are used interchangeably but strictly speaking, the ''suite'' is the definition of the communication protoc ...

used as part of call setup to establish a switched

virtual circuit A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is established within the network between two endpoints. The network, rather than having a fixed data rate reservation ...

between

Public Data Network A public data network (PDN) is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public. The first public p ...

s (PDNs), connecting two

network user address Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

es (NUAs). It consists of a maximum of fourteen binary-coded decimal digits and is sent over the

Packet Layer Protocol Packet Layer Protocol or PLP is the Network Layer protocol for the X.25 protocol suite. PLP manages the packet exchanges between DTE (data terminal) devices across VCs (virtual calls). PLP also can be used on ISDN Integrated Services Digital ...

(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 DNIC has three digits to identify the country (one to identify a zone and two to identify the country within the zone) and one to identify the PDN (allowing only ten in each country). The NTN identifies the exact network device (DTE,

data terminal equipment Data terminal equipment (DTE) is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment (DCE) ...

) in the

packet-switched network In telecommunications, packet switching is a method of grouping data into '' packets'' that are transmitted over a digital network. Packets are made of a header and a payload. Data in the header is used by networking hardware to direct the pack ...

(PSN) and is often provided as an NUA. There are no rules to the structure of the NTN. IPv4 addresses can be mapped to X.121 as described in RFC 1236. 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 IPv4 address exhaustion is the depletion of the pool of unallocated IPv4 addresses. Because the original Internet architecture had fewer than 4.3 billion addresses available, depletion has been anticipated since the late 1980s, when the Interne ...

References

See also