Content

| 7.1 |  | IS-IS Fundamentals |
| 7.1.2 |  | OSI terminology |

In an OSI network, four significant architectural entities exist. The entities are hosts, areas, a backbone, and a domain. A domain is any portion of an OSI network that is under a common administrative authority. Within any OSI domain, one or more areas can be defined. An area is a logical entity. An area is formed by a set of contiguous routers and the data links that connect them. All routers in the same area exchange information about all the hosts that they can reach. The areas are connected to form a backbone. All routers on the backbone know how to reach all areas. The term end system (ES) refers to any non-routing host or node. The term intermediate system (IS) refers to a router. These terms are the basis for the OSI ES-IS and IS-IS protocols.

A 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. The last byte in an NSAP identifies a process on the device, similar to a port or socket in TCP/IP.

A network entity title (NET) is an NSAP where the last byte is 0 (zero). The NET is used to identify a device. Therefore, each router would have a unique NET address.

A SNPA is the point at which subnetwork services are provided. This is the equivalent of the Layer 2 address corresponding to the Layer 3, NET or NSAP, address. This is usually a MAC address on a LAN or Virtual Circuit ID in X.25, Frame Relay, or ATM.

A circuit is an interface. A link is the path between two neighbor ISs and is defined as being "up" when communication is possible between the two neighbors’ SNPAs.