NSAP Addresses

NSAP Addresses

Network Service Access Point (NSAP) address aka ISO address is the network-layer address for CLNS packets (Connectionless Network Service- CLNS is similar to IP Service; a CLNS entity communicates using CLNP protocol with peer CLNS entity).

NSAP addresses are subdivided into two parts- Initial Domain Part (IDP) and Domain Specific Part (DSP).

The IDP consists of-

AFI- Authority and Format Identifier (1-byte)

IDI- Initial Domain Identifier (variable length)

The DSP consists of-

HO-DSP- High-Order of DSP

ID- System Identifier (6-bytes)

SEL- NSAP Selector (1-byte)

The HO-DSP may use any format as defined by the authority identified by IDP. The combination of [IDP, HO-DSP] identify both the routing domain and the area within the routing domain. Hence the combination [IDP, HO-DSP] is called the "Area Address". All nodes within the area must have same Area address.

The AFI has a binary value between 0 and 99; this value identifies the IDI and DSP format. AFI set to 49 indicates private address space.

A Network Entity Title (NET) is an NSAP address with NSEL set to 0. The NSAP address for all routers (ISes) are set with NSEL equal to 0.

A router can be configured with multiple NETs in following cases-

a) During Area Address migration from some area address A to another area address B.

b) During merging of two areas A & B

c) During partitioning of area C into two areas A & B.

The following are sample NETs-

1) 49.0001.1111.1111.1111.00

Area address = 49.0001

System ID = 1111.1111.1111

NSEL = 00

2) 49.0001.1234.AAAA.AAAA.AAAA.00

Area address = 49.0001.1234

System ID = AAAA.AAAA.AAAA

NSEL = 00