Section 9.2. Building a Network

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9.2. Building a Network

One of the best ways to understand how modern networks work is to look at how they evolved. Imagine that two people in an office want to hook their computers together so that they can share data. The easiest way to do this is to connect a cable between their serial ports. This is the simplest form of local area network (LAN), and requires virtually no special software or hardware. When one computer wants to send information to the other, it simply sends it out of its serial port (Figure 9-1).

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Figure 9-1. The simplest LAN

9.2.1. Ethernets

To make things a little more interesting, let's assume that another person wants to tie into the other two guys' existing network. With three computers in the network, we need an addressing scheme so that the computers can be differentiated. We would also like to keep the number of connections down to a minimum. The most common implementation of this kind of LAN is called an Ethernet^®. Ethernet is a hardware standard defining cabling, signaling, and behaviors that allow data to pass across a length of wire. The format of this data is defined by network protocols that we'll look at a bit later. The Ethernet standard was originally developed by Xerox Corporation and works like this:

• Each computer contains an Ethernet card, which is a special piece of hardware that has a unique Ethernet address.

• Every computer's Ethernet card is connected to the same single piece of wire.

• When a computer wishes to send a message to another computer with a particular Ethernet address, it broadcasts the message onto the Ethernet together with Ethernet header and trailer information that contain the Ethernet destination address. Only the Ethernet card whose address matches the destination address accepts the message.

• An attempt by two computers to broadcast to the Ethernet at the same time is known as a collision. When a collision occurs, they both wait a random period of time and then try again.

Figure 9-2 is a diagram of an Ethernet.

Figure 9-2. An Ethernet

Ethernet networks can transmit data in the order of tens or hundreds megabytes per second.

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9.2.2. Bridges

Let's assume that the Ethernet in the office works so well that the people in the office next door build themselves an Ethernet too. How does one computer on one Ethernet talk to another computer on another Ethernet? One solution might be to connect a special bit of hardware called a bridge between the networks (Figure 9-3). A bridge passes an Ethernet message between the different segments (wires) of the network as if both segments were a single Ethernet network cable. A bridge is used when you need to extend a network past the allowed length of a single section of wire (limited by signal degradation over a distance).

Figure 9-3. A bridge

9.2.3. Routers

The use of bridges facilitates the construction of small serially linked sections of Ethernet, but it's a pretty inefficient way to link together large numbers of networks. For example, assume that a corporation has four LANs that it wishes to interconnect in an efficient way. Stringing them all together with bridges would cause data to pass across the "middle" sections to get to the ends when hosts on those middle sections have no interest in the data. To pass data directly from the originating network to the destination network, a router can be used. A router is a device that hooks together two or more networks and automatically routes incoming messages to the correct network (Figure 9-4).

Figure 9-4. A router (This item is displayed on page 339 in the print version)

9.2.4. Gateways

The final stage in network evolution occurs when many corporations wish to connect their local area networks together into a single, large wide area network (WAN). To do this, several high-capacity routers called gateways are placed throughout the country, and each corporation ties its LAN into the nearest gateway (Figure 9-5).

Figure 9-5. Gateways (This item is displayed on page 339 in the print version)