Overview of Internetworking

Networking in the TCP/IP world consists of connecting different networks so that they form one logical interconnected network. This large overall network is called an internetwork, or more commonly, an internet. Each network uses its own physical layer, and the different networks are connected to each other by means of machines that are called internet gateways or simply gateways.

Note: This definition of a gateway is very different from the one used in general network terms where it is used to describe the function of a machine that links different network architectures. For example, a machine that connects an OSI network to an SNA network would be described as a gateway. Throughout this chapter, the TCP/IP definition of a gateway is used.

Figure 1 shows a simple internet with a gateway.

Figure 1. Networks with a Gateway Forming an Internet

The function provided by these gateways is to transfer IP datagrams between the 2 networks. This function is called routing and because of this the internet gateways are often called routers. Within this chapter, the terms router and gateway are synonymous; both refer to a machine that transfers IP datagrams between different networks.

The linking of the networks in this way takes place at the International Organization for Standardization (ISO) network level. It is possible to link networks at a lower layer level using bridges. Bridges link networks at the ISO data link layer. Bridges pass packets or frames between different physical networks regardless of the protocols contained within them. An example of a bridge is the IBM® 8209, which can interconnect an Ethernet network and a Token-Ring network.

Note: A bridge does not connect TCP/IP networks together. It connects physical networks together that will still form the same TCP/IP network. (A bridge does not do IP routing.)

Figure 2 depicts a router and a bridge. The router connects Network 1 to Network 2 to form an internet.

Figure 2. Routers and Bridges within an Internet