Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

The OSI (Open Systems Interconnection) model is a standard defined by the International Organization for Standardization. It is a layered architecture in which each layer defines a specific type of communication. The bottom layer, called the physical layer, is responsible for transmitting these messages as bit streams across a physical medium. The layers immediately above the physical layer define how data is packaged for transport over the physical network. Further up the protocol stack are layers that define how sessions between computers are established and managed. The uppermost layers define how applications interface with the network. The OSI model helps developers create products that work over a large variety of platforms and operating systems.

The network layer is the third layer of the protocol stack, just above the physical and data link layers. It is the routing layer and the layer that is responsible for network addressing. In the Internet protocol suite, IP resides in the network layer. Note the following:

• The network layer is also called the internetwork layer because it provides the functionality that allows different types of networks to be joined and share a common addressing scheme. See "Internetworking."


• IP (Internet Protocol) is the most common network layer protocol. Others include Novell's IPX and IBM's APPN (Advanced Peer-to-Peer Networking). See "IP (Internet Protocol)."


• Network layer protocols offer best-effort services, as opposed to transport layer services, which provide reliable data delivery services. See "Best-Effort Delivery" and "Reliable Data Delivery Services."


• Network layer protocols such as IP are connectionless, as opposed to transport layer services, which are connection-oriented. See "Connection-Oriented and Connectionless Services."

A common way to think of the network layer is as a service provider to the transport layer. In the TCP/IP protocol suite, IP provides connectionless (unreliable) packet delivery services, while TCP provides transport layer, connection-oriented services. See "TCP (Transmission Control Protocol)" and "Transport Protocols and Services" for more information about the transport layer.

RFC 2956 (Overview of 1999 IAB Network Layer Workshop, October 2000) describes the "state of the network layer and its impact on continued growth and usage of the Internet." In particular, it discusses NAT (network address translation), firewalls, IPv6, addressing, and various routing issues.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.