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| 7.2 |  | ISO Addressing |
| 7.2.1 |  | NSAPs |

OSI network layer addressing is implemented by using two types of hierarchical addresses, NSAP and NET.

The NSAP is a conceptual point on the boundary between the network and the transport layers. The NSAP is the location at which OSI network services are provided to the transport layer. Each transport layer entity is assigned a single NSAP. The NSAP address identifies any system in an OSI network. Various NSAP formats are used for various systems. Different protocols may use different representations of the NSAP.

The NSAP address is the network layer address for CLNS packets. As with DECnet Phase V, one NSAP address is used for each device, not for each interface. LSPs, Hello PDUs, and other routing PDUs are OSI formatted PDUs. Therefore, every IS-IS router requires an OSI address. IS-IS uses the OSI address in the LSPs to identify the router, build the topology table, and build the underlying IS-IS routing tree. NSAP addresses contain the OSI address of the device and provide a link to upper-layer processes. The NSAP address can be thought of as equivalent to the combination of an IP address and upper-layer protocol identifier in an IP header.

An NSAP address consists of three parts. They are the area address, the system ID, and the NSAP selector byte. The total length is between eight and 20 bytes. The area address is a variable length field composed of high order octets, excluding the system ID and the selector byte. The system ID is the ES or IS identifier in an area, similar to the OSPF router ID. The system ID has a fixed length of six bytes as engineered in the Cisco IOS. The N-selector byte is a service identifier. The role of N-selector byte is analogous to that of a port or socket in TCP/IP.

Cisco routers can route CLNS data that uses addressing conforming to the ISO 10589 standard. The fields specified in this standard are as follows:

- Authority and Format ID (AFI) – One byte, actually a binary value between 0 and 99, used to specify the IDI format and DSP syntax of the address and the authority that assigned the address.
- Inter-Domain ID (IDI) – Identifies the domain.
- Inter-Domain Part (IDP) – Consists of the AFI and IDI together. This is roughly equivalent to a classful IP network, in decimal format.
- High-Order DSP (HODSP) – Used for subdividing the domain into areas. This is roughly equivalent to a subnet in IP.
- System ID – Identifies an individual OSI device. In OSI, a device has an address, just as it does in DECnet, while in IP an interface has an address.
- NSAP-Selector (NSEL) – Identifies a process on the device. It is roughly equivalent to a port or socket in TCP/IP. The NSEL is not used in routing decisions.
- Domain-Specific Part (DSP) – Comprised of the HODSP, the system ID, and the NSEL in binary format.

IS-IS uses a simple two-layer architecture. IS-IS joins the IDP and HODSP together and treats them as the Level 2 area ID, with the remaining system ID used for Level 1 routing. Restated, in IS-IS, everything to the left of the system ID is used as the area ID. The minimum length of this area ID is a single byte. The maximum is the remaining 13 bytes permitted by the ISO standard. Therefore, an NSAP for an IS-IS network could be as little as eight bytes in length. The length is normally longer so as to permit some granularity in the allocation of areas.

There are three NSAP formats. The first is a simple 8-byte area ID and system ID format. The second is an OSI NSAP format, and the third is a Government OSI Profile (GOSIP) NSAP format. Cisco supports all NSAP formats that are defined by ISO 8348/Ad2, which are described in this section.