X.121
Encyclopedia
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 Network
s (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 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
) in the packet-switched network (PSN) and is often provided as an NUA. There are no rules to the structure of the NTN.
IP 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.
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
address format of the X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
protocol suite used as part of call setup to establish a switched virtual circuit
Virtual circuit
In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
between Public Data Network
Public Data Network
A public data network 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....
s (PDNs), connecting two network user addresses (NUAs). It consists of a maximum of fourteen binary-coded decimal
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...
digits and is sent over the Packet Layer Protocol
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 devices across VCs...
(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
Data Terminal Equipment 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...
) in the packet-switched network (PSN) and is often provided as an NUA. There are no rules to the structure of the NTN.
IP 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.