When configuring ISIS, under the router process, it mandatory to have a NET statement, for example:

router (config) # router isis 1

router (config-isis) # net 49.0002.3131.3131.3131.00

So what is the NET statement and what do it’s segments mean?

The Network Entity Title (NET) is an Network Service Acces Point (NSAP) address whose trailing byte (NSEL) is set to 0. Defined in ISO/IEC 8348, an NSAP represents a service in a particular system (node) in a domain
(autonomous system or AS), and gives info about the AS, areas, node itself and even service. It can be a variable length between 8-20 bytes. No two nodes in a broadcast domain can have the same system id.

So what is the format of the NET. The Cisco Live talk by Elvin Arias gave an excellent breakdown of how the NET is made up:

The Authority And Format Identifier (AFI) identifies the overall format of the NSAP, with two most common ones: AFI 49 (private) or 47 (International Code Designator).

Then in order, the Initial Domain Identifier (IDI) which highlights the top level domain(s), High Order Domain Specific Part (HO-DSP) specifies the actual domain, and Network Selector (NSEL) which is the system id itself. This is followed by the Network Selector (NSEL), where ’00’ indicates just the node itself.

The Cisco deployment of ISIS requires at least 8 bytes for the NET address:

The dots are arbitary and can be added if not added will not affect operations. It is possible to have multiple NET statements, but it is recommended to have only one. In order to check the NET id of a particular ISIS process, use:

router # show isis protocol