X.121 Specification for X.25 Address Structure (123203)

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The following text explains the structure of the X.25 address.

Data Network Identification Code (DNIC)

The DNIC makes up the first four digits in an X.121 or X.25 address.

Country Code

The first three digits in the DNIC are the country code. For example, the country code for Canada is 302, so every X.25 network address in Canada begins with 302. The U.S. has several country codes. U.S. DNICs can start with 310, 311, 312, or 313.

Network ID

The fourth digit is the Network ID, identifying which network it is. For example, DataPac of Canada has a network ID of 0, so the DNIC for DataPac is 3020. Infoswitch of Canada has a network ID of 9, so Infoswitch has a DNIC of 3029. In the U.S. MCI has a DNIC of 3106, Sprinet has a DNIC of 3110, and Infonet has a DNIC of 3137.

The National Number

The National Number is the 5th through 12th digits of the X.25 address. This number is used to identify a unique DCE node (X.25 node) within a network. Network vendors lease DCE or X.25 nodes. Each of these nodes must have a unique X.121 address. Within a given vendor's X.25 network, the first four digits of the DCE addresses will all be the same--they will be the DNIC for the vendor. Digits 5 through 12 (the National Number) are assigned by the network vendor to create a unique X.25 address for each X.25 node, or Circuit Terminating Equipment (DCE).

Subaddress

Most networks support a two-digit subaddress, which can be added to the end of the National Number. The two digits are not used by the network, but are passed through for use by the user's Data Terminating Equipment (DTE). The DTE is primarily the X.25 card (Eicon or Atlantis) that the X.25 application uses to communicate with the network DCE node, where each physical port on an X.25 card is connected to a DCE node.

The two subaddress digits are usually used when X.25 applications are located on LAN workstations. Eicon and Atlantis cards require a two-digit subaddress when they will be managing X.25 sessions (or virtual circuits) for X.25 applications (such as an MTA or X.400 Gateway) when the application is running on a LAN workstation that does not contain the Eicon or Atlantis card.

Note that the .INI entry "X25SubAddress=" for MTA in PC Mail version 3.2x is somewhat misleading because when the .INI also has the entry "CommType=X25Eicon", the "X25SubAddress" will always be the X.25 address of the network DCE the Eicon port is attached to (usually a 12-digit number composed of DNIC and a National Number), with the actual two-digit subaddress appended to the end of the long X.25 (X.121) address if it is needed. For MTAs launched with the INI entry "CommType=X25Atlan", the .INI entry "X25SubAddress=" is only two digits. The difference is due to design differences between the Ecion and Atlantis cards.

Inter-Network

Sometimes to make a connection between two networks in two different countries, an extra digit is added before the DNIC, usually a one. This varies by vendor.

The current maximum length for an X.121 address is 15 digits. The typical address length is 12 or 14 digits.

Null-Modem Operation

When two X.25 cards are connected with an X.25 null-modem cable, there is no network present and the long X.121 address is not needed. The optional two-digit subaddress will be needed if the X.25 cards are also connected to a LAN and will be establishing X.25 sessions for PC Mail version 3.2x MTAs, Gateways or MS Exchange processes running on LAN PCs, PCs without the X.25 card installed in them.