Addresses in the ISO network architecture are referred to as network service access point (NSAP)addresses and network entity
titles (NETs). Each node in an OSI network has one or more NETs. In addition, each node has many NSAP addresses. Each NSAP
address differs from one of the NETs for that node in only the last byte. This byte is called the N-selector. Its function
is similar to the port number in other protocol suites.
Our implementation supports all NSAP address formats that are defined by ISO 8348/Ad2; however, Cisco provides ISO Interior
Gateway Routing Protocol (IGRP) or Intermediate System-to-Intermediate System (IS-IS) dynamic routing only for NSAP addresses
that conform to the address constraints defined in the ISO standard for IS-IS (ISO 10589).
An NSAP address consists of the following two major fields, as shown in Figure 1:
The initial domain part (IDP) is made up of 1-byte authority and format identifier (AFI) and a variable-length initial domain
identifier (IDI). The length of the IDI and the encoding format for the domain specific part (DSP) are based on the value
of the AFI.
The DSP is made up of a High Order DSP (HO-DSP), an area identifier, a system identifier, and a 1-byte N-selector (labeled
Assign addresses or NETs for your domains and areas. The domain address uniquely identifies the routing domain. All routers
within a given domain are given the same domain address. Within each routing domain, you can set up one or more areas, as
shown in Figure 2. Determine which routers are to be assigned to which areas. The area address uniquely identifies the routing
area and the system ID identifies each node.
The key difference between the ISO IGRP and IS-IS NSAP addressing schemes is in the definition of area addresses. Both use
the system ID for Level 1 routing (routing within an area). However, they differ in the way addresses are specified for area
routing. An ISO IGRP NSAP address includes three separate fields for routing: the domain, area, and system ID. An IS-IS address
includes two fields: a single continuous area field (comprising the domain and area fields) and the system ID .