IS-IS Overview https://www.youtube.com/watch?v=78lFGoVllc0 https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/concept/is-is-routing-overview.html

• NETs take several forms, depending on your network requirements. NET addresses are hexadecimal and range from 8 octets to 20 octets in length. Generally, the format consists of an authority and format Identifier (AFI), a domain ID, an area ID, a system identifier, and a selector. The simplest format omits the domain ID and is 10 octets long. For example, the NET address 49.0001.1921.6800.1001.00 consists of the following parts:
• 49—AFI
• 0001—Area ID
• 1921.6800.1001—System identifier
• 00—Selector

Example Net Address: 49.0001.1921.6800.1002.00

The IS-IS address consists of three parts:

Area identifier: The first three bytes are the area ID. The first byte of this example — 49 — is the address family identifier (AFI) of the authority, which is equivalent to the IP address space that is assigned to an autonomous system. The AFI value 49 is what IS-IS uses for private addressing, which is the equivalent of RFC 1918 address space for IP protocols.

The second two bytes of the area ID — 0001 — represent the IS-IS area number. In this example, the area number is 1.

System identifier: The next six bytes identify the node (that is, the router) on the network. The system identifier is equivalent to the host or address portion on an IP address.

Although you can choose any value for the system identifier, a commonly used method is to use binary-coded decimal (BCD) which involves taking the router’s IP address (the address you assigned to the lo0 loopback interface), filling in all leading zeros, and then repositioning the decimal points to form three two-byte numbers.

In this example, if you pad the IP address 192.168.1.2 with zeros, the result is 192.168.001.002. Rearranging the decimal points gives you 1921.6800.1002.

NET selector: The final two bytes are the NET selector (NSEL). For IS-IS, they must always be 00, to indicate “this system.