End to End communication requires addressing on the network layer. OSI uses an addressing known as Network Service Access Point addressing (NSAP).
NSAP has many differences with TCP/IP addressing, NSAP addresses are assigned to an entier network node, not to individual interfaces. A single node requires only one NSAP address, regardless of how many network interfaces it uses. NSAP does not use subnets like IP does.
NSAP addresses consist of two parts:
- Initial Domain Part (IDP)
- Domain Specific Part (DSP)
NSAP addresses are variable length. The IDP itself consists of two fields, the Authority and Format Identifier (AFI) and the Initial Domain Identifier (IDI). The AFI value can be between 00 and FF and indicates the format of the remaining address fields.
The IDI field is variable length depending on the AFI and may even be omitted in some instances. Together the AFI and IDI indicate the routing domain (or AS) in which the node is located.
The DSP is dependent on the AFI, however the DSP consists of a variable length High Order Domain Specific Part (HO-DSP) which identifies the part (or area) of the domain in which the node is located.
The HO-DSP can be structured into subfields, the System ID is the unique identifier of the node itself. All current implementations fix the length of the System ID to 6 octects. The SEL fields also called the NSAP Selector or NSEL is 1 octect long and identifies the service in or above the network layer.