Overview

A computer can communicate directly only with computers with which it shares a physical connection. Given this fact, the computer on your desk should be able to communicate only with computers that are electrically connected to the network cable that connects to your system. So, how does it communicate with a computer on the other side of the world? There are two primary techniques: circuit switching and packet switching.

Circuit switching is the technique used by the voice telephone network. When you pick up the telephone, you hear a dial tone. At this point, you have an electrical connection to the telephone switch at the local telephone company's central office. As you dial the telephone number, you provide the switch with the information it needs to make additional connections. Using this information, the switch connects your inbound port to an outbound port. If the number you are calling is serviced by the local switch, it sets up a connection between the port your telephone is attached to and the port connected to the phone you're calling. If the number you are calling is remotely located, the local switch sets up a connection to the next switch down the line. Each switch connects to the next switch in line until the switch servicing the remote phone is reached. This creates a circuit from your phone to the remote phone, wherever it is located, which is dedicated to your use until you hang up the phone. When your computer communicates over a modem, it uses the telephone system to create a circuit between itself and the remote system, which is often the server at an ISP that connects your system into the packet-switched Internet.

Packet switching is the technique used by most data networks. Every packet in the network contains an address that tells the switch where the packet is bound. When the packet arrives at a switch, the switch reads the address and determines how the packet should be forwarded. If the switch has a physical connection to the destination node, it delivers the packet itself. Otherwise, it forwards the packet to the next switch in the path toward the destination node. Each packet is handled separately. No end-to-end connection is established

In the circuit-switched model, the connection is between your phone and the phone at the remote end. In the packet-switched system, the connection is between your host and the local router. Figure 7.1 illustrates that packet switches use hop-by-hop routes versus the end-to-end connections used by circuit switches.

Circuit Switching Packet Switching

Figure 7.1: Circuit switching versus packet switching

Note Whether you configure your system as a host or as an IP router, it will not have end-to-end knowledge of the routes through the network. It will know only about local routers. The Internet—and all TCP/IP networks—are packet-switched networks. An IP packet switch is called a gateway or an IP router. Routers interconnect networks, moving data from one network to another until the destination network is reached. At that point, direct delivery is made to the destination host. This is illustrated in Figure 7.2.

Figure 7.1: Circuit switching versus packet switching

Note Whether you configure your system as a host or as an IP router, it will not have end-to-end knowledge of the routes through the network. It will know only about local routers. The Internet—and all TCP/IP networks—are packet-switched networks. An IP packet switch is called a gateway or an IP router. Routers interconnect networks, moving data from one network to another until the destination network is reached. At that point, direct delivery is made to the destination host. This is illustrated in Figure 7.2.

Bear Walrus

Figure 7.2: Routing through networks

In the figure, an IP datagram from finch to walrus would first go to router A, then to router B, then to router C, and finally to walrus. Notice that IP routers can interconnect different types of physical networks. And, as this chapter demonstrates, any Linux system can be configured to be an IP router.

Bear Walrus

Figure 7.2: Routing through networks

In the figure, an IP datagram from finch to walrus would first go to router A, then to router B, then to router C, and finally to walrus. Notice that IP routers can interconnect different types of physical networks. And, as this chapter demonstrates, any Linux system can be configured to be an IP router.

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