you can find this in product documenation.
IS-IS Address Structure
The network service access point (NSAP) is an address defined by the OSI to locate resources. Figure 4 shows the NSAP address structure. The NSAP is composed of the initial domain part (IDP) and the domain specific part (DSP). The lengths of the IDP and the DSP are variable. The maximum length of the NSAP is 20 bytes and its minimum length is 8 bytes. •The IDP is similar to the network ID in an IP address. It is defined by the ISO and consists of the authority and format identifier (AFI) and the initial domain identifier (IDI). The AFI indicates the address allocation authority and address format, and the IDI identifies a domain.
•The DSP is similar to the subnet ID and host address in an IP address. The DSP consists of the High Order DSP (HODSP), system ID, and NSAP Selector (SEL). The HODSP is used to divide areas, the system ID identifies a host, and the SEL indicates the service type.
Figure 4 IS-IS address structure
•Area Address
The IDP and the HODSP of the DSP identify a routing domain and the areas in a routing domain. Therefore, the combination of the IDP and HODSP is called an area address, which is similar to an area number in OSPF. The area addresses of routers in the same Level-1 area must be the same, while the area addresses of routers in the Level-2 area can be different.
In general, a router can be configured with only one area address. The area address of all nodes in an area must be the same. In the implementation of a device, an IS-IS process can be configured with a maximum of three area addresses to support seamless combination, division, and transformation of areas.
•System ID
A system ID uniquely identifies a host or a router in an area. In the device, the fixed length of the system ID is 48 bits (6 bytes).
In actual applications, a router ID corresponds to a system ID. If a router takes the IP address 168.10.1.1 of Loopback 0 as its router ID, its system ID used in IS-IS can be obtained in the following way:
◾Extend each part of IP address 168.10.1.1 to 3 bits and add 0 to the front of any part that is shorter than 3 bits. Then the IP address is extended as 168.010.001.001.
◾Divide the extended address 168.010.001.001 into three parts, each of which consists of four decimal digits. Then system ID 1680.1000.1001 is obtained.
You can specify a system ID in many ways. You need to ensure that the system ID uniquely identifies a host or a router.
•SEL
The role of an SEL is similar to that of the "protocol identifier" of IP. A transport protocol matche*******L. The SEL is always "00" in IP.
A Network Entity Title (NET) indicates the network layer information of an IS itself and consists of an area ID and a system ID. It does not contain the transport layer information (SEL = 0). A NET can be regarded as a special NSAP. The length of the NET field is the same as that of an NSAP. Its maximum length is 20 bytes and its minimum length is 8 bytes. When configuring IS-IS on a router, you can configure only a NET instead of an NSAP.
Assume that there is a NET: ab.cdef.1234.5678.9abc.00. In the NET, the area address is ab.cdef, the system ID is 1234.5678.9abc, and the SEL is 00.