Layer 3: The Network Layer

The Network Layer is responsible for establishing the route to be used between the originating and destination computers. This layer lacks any native transmission error detection/correction mechanisms and, consequently, relies upon the link for reliable transmission service of the Data Link Layer. End-to-end reliability across several physical links is more of a function of the Transport Layer (Layer 4).

The Network Layer is used to establish communications with computer systems that lie beyond the local LAN segment. It can do so because it has its own routing addressing architecture, which is separate and distinct from the Layer 2 machine addressing. Such protocols are known as routed or routable protocols. Routable protocols include IP, Novell’s IPX, and AppleTalk, although this book will focus exclusively on IP and its related protocols and applications.

Use of the Network Layer is optional. It is required only if the computer systems reside on different network segments that are separated by a router, or if the communicating applications require some service, feature, or capability of either the Network Layer or the Transport Layer. For example, two hosts that are directly connected to the same LAN may communicate well using just that LAN’s communications mechanisms (Layers 1 and 2 of the OSI Reference Model).

In modern networks, the requirement to connect two networks is so common that the Network Layer is no longer considered optional. Earlier protocols such as NetBEUI that do not provide an explicit network layer have to be handled as special cases in modern network design consisting of interconnected networks.