In an IS-IS network, there are End Systems, Intermediate Systems, Areas and Domains. End systems are user devices. Intermediate systems are routers. Routers are organized into local groups called "areas", and several areas are grouped together into a "domain". IS-IS is designed primarily providing intra-domain routing or routing within an area. IS-IS, working in conjunction with CLNP, ES-IS, and IDRP, provides complete routing over the entire network.

IS-IS routing makes use of two-level hierarchical routing. Level 1 routers know the topology in their area, including all routers and hosts, but they do not know the identity of routers or destinations outside of their area. Level 1 routers forward all traffic for destinations outside of their area to a level 2 router within their area which knows the level 2 topology. Level 2 routers do not need to know the topology within any level 1 area, except to the extent that a level 2 router may also be a level 1 router within a single area.

IS-IS has been adapted to carry IP network information, which is called Integrated IS-IS. Integrated IS-IS has the most important characteristic necessary in a modern routing protocol: It supports VLSM and converges rapidly. It is also scalable to support very large networks.

There are two types of IS-IS addresses:

Network Service Access Point (NSAP) - NSAP addresses identify network layer services, one for each service running.

Network Entity Title (NET) - NET addresses identify network layer entities or processes instead of services.

Devices may have more than one of each of the two types of addresses. However NET"s should be unique, and the System ID portion of the NSAP must be unique for each system.

• Intradomain routing protocol discriminator - Network layer protocol identifier assigned to this protocol
• Length indicator - Length of the fixed header in octets.
• Version/protocol ID extension - Equal to 1.
• ID length - Length of the ID field of NSAP addresses and NETs used in this routing domain.
• R - Reserved bits.
• PDU type - Type of PDU. Bits 6, 7 and 8 are reserved.
• Version - Equal to 1.
• Maximum area addresses - Number of area addresses permitted for this intermediate systems area.

Format of NSAP for IS-IS:

• IDP - Initial Domain Part
• AFI - Authority and Format Identifier (1-byte); Provides information about the structure and content of the IDI and DSP fields.
• IDI - Initial Domain Identifier (variable length)
• DSP - Domain Specific Part
• HO-DSP - High Order Domain Specific Part
• Area Address (variable)
• ID - System ID 1- 8 bytes
• SEL - n-selector (1-byte value that serves a function similar to the port number in Internet Protocol).