An Overview of X.25 Addressing

ITU (CCITT) Suggestion

The International Telephone Union (ITU) has suggested a format for addresses in public data networks (PDNs) in its Recommendation X.121.

This recommendation states that an address in the PDN should consist of 5 to 14 digits. The first four digits are called the DNIC (data network identification code).

The first three digits of the DNIC identify the country where the PDN is located, and the fourth digit identifies the specific PDN in that country.

For example, 3110 and 3108 are two DNICs used by two PDNs in the United States and 2080 is the DNIC used for the French PDN. 311 and 310 are country codes identifying the United States; 208 is the country code for France.

The DNIC is followed by an NTN (network terminal number), which specifies the destination on the PDN.

The NTN can consist of from 1 to 10 digits; the X.121 specification does not provide any further details. This means that each country can decide how many digits will make up the NTN. This number frequently corresponds to the number of digits used in a national telephone number.

In addition to the DNIC + NTN, Recommendation X.121 leaves open the possibility of including a 1-digit prefix (normally 0 or 1) in front of the address. This prefix is not considered part of the address.