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.
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