All internets contain three elements: hosts, networks, and gateways.
A host is a computer or device that is part of an internet. This includes a:
Hosts on an internet can use different operating systems. The TCP/IP protocols give each operating system a set of shared rules to follow for internet communications. Figure 1 shows an internet created from different hosts that share the same network.
A network consists of two or more hosts connected by a single communications medium, such as Ethernet or X.25. Two common types of networks are:
One goal of an internet is to connect varying types of networks independently of their underlying hardware technologies. One way to resolve hardware differences is through the use of gateways.
A gateway (or router) is a device that connects two or more networks to form an internet. A gateway can be a dedicated computer that resolves hardware differences between networks and routes packets of data between the networks it connects. A host can serve as a gateway while providing other services to its users. (See Chapter IV for information about routing.)
Figure 2 shows an internet consisting of two networks connected by a gateway. Gateway Homer allows the hosts on Network 1 to communicate with the hosts on Network 2. The two networks do not have to use the same hardware interfaces. The gateway resolves all differences in connectivity.