ISO network layer addresses are flexible enough to make routing feasible in a worldwide Internet. Network layer addresses in ISO and IP are hierarchical and clearly identify level 1 and level 2 areas. These addresses can be up to 20 octets long; any packet that contains an address has one additional octet to specify the length of the address.
An ISO address—also known as the NSAP address—is broken into three parts: the area address, the system identifier (ID), and the NSAP selector.
area address
system ID
selector
The area address defines the routing domain and the area within the routing domain. The length of the ID field can be from 1 to 8 octets and uses a single fixed length for any one routing domain. The selector field is always 1 octet long. Usually, all end systems within the same area have the same area address. Some areas can have multiple addresses. The NSAP address is defined by the network entity title (NET) during configuration.
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