7.1 X.25
7.1.5 X.121 - The X.25 addressing standard
The format of X.25 addresses is defined by the ITU-T X.121 standard:
  • The first four digits specify the Data Network Identification Code (DNIC). The first three digits specify the country code. The fourth digit is the provider number assigned by the ITU-T. Countries that require more than ten provider numbers are assigned multiple country codes. The United States, for example, is assigned country codes 310 through 316 to represent the country code, and the fourth digit is assigned by the ITU-T.
  • The remaining 8 to 10 or 11 digits specify the network terminal number (NTN) that is assigned by the packet-switched network (PSN) provider.

For your specific DNIC code, consult your service provider. For a listing of ITU-T country code assignments, refer to the ITU-T Recommendation X.121. The ITU-T Wet site is www.itu.org. A sample of the DNIC country codes is listed in Table .

For different network protocols to connect across X.25, statements are entered on the router to map the next-hop network-layer address to an X.121 address. For example, an IP network-layer address is mapped to an X.121 address to identify the next-hop host on the other side of the X.25 network.

These statements are logically equivalent to the LAN Address Resolution Protocol (ARP) that dynamically maps a network-layer address to a data-link Media Access Control (MAC) address. Maps are required for each protocol because ARP is not supported in an X.25 network. 

Unlike the ARP that dynamically maps a network-layer address, static X.25 map statements must be configured manually.