The CCITT document X.121 defines an addressing scheme for phone numbers. That same scheme is used for X.25 addressses. All X.25 packets begin with a 3 byte header composed of a group, channel and type field. The GROUP byte always begins with the sequence 0001. The GROUP and CHANNEL bytes are there to provide a 12 bit field for the X.121 address.
X.121 addresses are used by the X.25 Packet Layer Protocol (PLP) in call-setup mode to establish SVCs. The X.121 address includes an IDN field. The X.121 Address field includes the International Data Number (IDN), which consists of two fields: the Data Network Identification Code (DNIC) and the National Terminal Number (NTN). DNIC is an optional field that identifies the exact PSN in which the destination DTE device is located. This field is sometimes omitted in calls within the same PSN. The DNIC has two subfields: Country and PSN. The Country subfield specifies the country in which the destination PSN is located. The PSN field specifies the exact PSN in which the destination DTE device is located. The NTN identifies the exact DTE device in the PSN for which a packet is destined. This field varies in length.
The X.121 Address field includes the International Data Number (IDN), which consists of two fields: the Data Network Identification Code (DNIC) and the National Terminal Number (NTN). DNIC is an optional field that identifies the exact PSN in which the destination DTE device is located. This field is sometimes omitted in calls within the same PSN. The DNIC has two subfields: Country and PSN. The Country subfield specifies the country in which the destination PSN is located. The PSN field specifies the exact PSN in which the destination DTE device is located. The NTN identifies the exact DTE device in the PSN for which a packet is destined. This field varies in length.