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| 7.2 |  | ISO Addressing |
| 7.2.2 |  | NETs |

An NSAP address with an NSEL value of 00 is used to identify the device itself, which is the network address of the device. In this case, the NSAP is known as a NET. So a NET is determined by the area ID and system ID.

All router NETs have an N-selector of 00, implying the network layer of the IS itself. The 00 indicates there is no transport layer entity associated with this address. For this reason, the NSAP of a router is always referred to as a NET. The NSEL is like a TCP port number.

Routers are identified with NETs of 8 to 20 bytes. ISO/IEC 10589 distinguishes only three fields in the NSAP address format. These are a variable-length area address beginning with a single octet, a system ID, and a 1-byte N-selector. Cisco implements a fixed length of six bytes for the system ID, which is similar to the OSPF router ID.

In general, the big difference between NSAP style addressing and IP style addressing is that there will be a single NSAP address for the entire router. However, with IP there will be one IP address per interface.

The following are some guidelines for NETs:

- All ISs and ESs in a routing domain must have system IDs of the same length.
- All routers in an area must have the same area address.
- All Level 2 routers must have a unique system ID domain wide.
- All Level 1 routers must have a unique system ID area wide.
- All ESs in an area will form an adjacency with a Level 1 router on a shared media segment if they share the same area address.
- If multiple NETs are configured on the same router, they must all have the same system ID.

The NET is used by routers to identify themselves in the LSPs and forms the basis for OSI route calculation. Addresses starting with value 49, AFI = 49, are considered as private addresses. Private addresses are analogous to those specified by RFC 1918 for IP addresses. These addresses are routed by IS-IS. However, this group of addresses should not be advertised to other CLNS networks.

Addresses starting with AFI values of 39 and 47, respectively, represent ISO Data Country Code and ISO International Code Designator. RFC 941 allocated NSAP addresses. The division of the global network addressing domain according to IDI format and the numbers adjacent to each line in Figure are AFI values.

It is possible to configure multiple NETs on a router, but no router is ever in more than one area. Configuring multiple NETs causes the areas to merge into a common area, leaking the Level 1 databases into each other. The only reasons to have multiple NETs are for splitting, merging, or renumbering areas. This method should only be used in periods of transition. Using multiple NETs is analogous to using secondary addresses with IP. Cisco limits the number of configurable NETs to three per router.