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The Network Service Access Point (NSAP) is one of two types of hierarchical addresses (the other type is the network entity title) used to implement Open Systems Interconnection (OSI) network layer addressing. The NSAP is the logical point between the network and transport layers where network services are delivered to the transport layer; the location of this point is identified to the OSI network service provider by the NSAP address. There are two NSAP address fields, Initial Domain Part (IDP) and the Domain-Specific Part (DSP).
The IDP consists of the Authority Format Identifier (AFI) and the Initial Domain Identifier (IDI). AFI serves to provide information about the makeup of IDI and DSP, indicating, for example, whether the DSP uses decimal or binary notation, or whether or not the IDI may be of variable length.
The DSP is made up of four fields: the Address Administration field, which enables further address administration tasks and task delegation; the Area field, which is used for routing and identifies the specific area within a domain; the Station field, also used for routing purposes, which specifies the station within the area; and the Selector field, also used for routing, which identifies a specific entity within the station called an n-selector, the last byte of the NSAP address. If an OSI end system has multiple NSAP addresses - which is frequently the case - they will typically differ only in the n-selector.
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