TCP/IP Defined - Elements of an Internet


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:

  • Multiuser system, such as a minicomputer
  • Single-user system, such as a microcomputer or personal computer
  • Workstation
  • Terminal server

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.

Figure 1: Internet Created from Different Hosts



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:

  • Local area network (LAN) — A LAN typically runs at high speeds and spans a small geographic area, such as a single room or building.
  • Wide area network (WAN) — Hosts in a WAN are connected through dial-up or leased lines (such as telephone systems). Although a WAN typically runs at much slower speeds than a LAN, it can transfer data across a large geographic distance, such as a state or continent.

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.


Gateways and Routers

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.

Figure 2: An Internet Created by Connecting Two Networks