Computer networks

Last updated: December 2, 2009.

Thank goodness for computer networks! If they'd never been invented, you wouldn't be reading this now (using the Internet) and I wouldn't be writing it either (using a wireless home network to link up my computer equipment). There's no doubt that computer networking is extremely complex when you delve into it deeply, but the basic concept of linking up computers so they can talk to one another is pretty simple. Let's take a closer look!

Photo: Testing a small computer network linked to the Internet. Photo courtesy of NASA Glenn Research Center (NASA-GRC).

What is a computer network?

You can do lots of things with a computer but, connect it up to other computers and peripherals (the general name given to add-on bits of computer equipment such as modems, inkjet and laser printers, and scanners) and you can do an awful lot more. A computer network is simply a collection of computer equipment that's connected with wires, optical fibers, or wireless links so the various separate devices (known as nodes) can "talk" to one another and swap data (computerized information).

Photo: From the outside, a computer network looks like nothing more than a bundle of wires. Photo courtesy of NASA Glenn Research Center (NASA-GRC).

Types of networks

Not all computer networks are the same. The network I'm using to link this laptop to my wireless router, printer, and other equipment is the smallest imaginable. It's an example of what's sometimes called a PAN (personal area network)—essentially a convenient, one-person network. If you work in an office, you probably use a LAN (local area network), which is typically a few separate computers linked to one or two printers, a scanner, and maybe a single, shared connection to the Internet. Networks can be much bigger than this. At the opposite end of the scale, we talk about MANs (metropolitan area networks), which cover a whole town or city, and WANs (wide area networks), which can cover any geographical area. The Internet is a WAN that covers the entire world but, in practice, it's a network of networks as well as individual computers: many of the machines linked to the Net connect up through LANs operated by schools and businesses.

Photo: A wireless router like this one, made by Netgear, is the heart of many home PANs.

Rules

Computers are all about logic—and logic is all about following rules. Computers networks are a bit like the army: everything in a network has to be arranged with almost military precision and it has to behave according to very clearly defined rules. In a LAN, for example, you can't connect things together any old how: all the nodes (computers and other devices) in the network have to be connected in an orderly pattern known as the network topology. You can connect nodes in a simple line (also called a daisy chain or bus), with each connected to the next in line. You can connect them in a star shape with the various machines radiating out from a central controller known as the network server. Or you can link them into a loop (generally known as a ring). All the devices on a network also have to follow clearly defined rules (called protocols) when they communicate to ensure they understand one another—for example, so they don't all try to send messages at exactly the same time, which causes confusion.

Artwork: The three best-known computer network topologies: line (chain/bus), ring, and star.

Permissions and security

Just because a machine is on a network, it doesn't automatically follow that every other machine and device has access to it (or can be accessed by it). The Internet is an obvious example. If you're online, you get access to billions of Web pages, which are simply files stored on other machines (servers) dotted all over the network. But you can't access every single file on every single computer hooked up to the Internet: you can't read my personal files and I can't read yours, unless we specifically choose for that to happen.

Permissions and security are central to the idea of networking: you can access files and share resources only if someone gives you permission to do so. Most personal computers that connect to the Internet allow outgoing connections (so you can, theoretically, link to any other computer), but block most incoming connections or prohibit them completely. Servers (the machines on the Internet that hold and serve up Web pages and other files) operate a more relaxed policy to incoming connections. You've probably heard of hacking, which, in one sense of the word, means gaining unauthorized access to a computer network by cracking passwords or defeating other security checks. To make a network more secure, you can add a firewall (either a physical device or a piece of software running on your machine, or both) at the point where your network joints onto another network or the Internet to monitor and prohibit any unauthorized, incoming access attempts.

What makes a network?

To make a network, you need nodes and connections (sometimes called links) between them. Linking up the nodes means making some sort of a temporary or permanent connection between them. In the last decade or so, wireless connections have become one of the most popular ways of doing this, especially in homes. In offices, wired connections are still more commonplace—not least because they are generally faster and more secure and because many newer offices have network cabling already in place.

Photo: If your laptop doesn't have a network card, you can simply plug in a PCMCIA adapter like this one. The adapter has a network card built into it.

Apart from computers, peripherals, and the connections between them, what else do you need? Each node on a network needs a special circuit known as a network card (or, more formally, a network interface card or NIC) to tell it how to interact with the network. Most new computers have network cards built in as standard. If you have an older computer or laptop, you may have to fit a separate plug-in circuit board (or, in a laptop, add a PCMCIA card) to make your machine talk to a network. Each network card has its own separate numeric identifier, known as a MAC (media access control) code or LAN MAC address. A MAC code is a bit like a phone number: any machine on the network can communciate with another one by sending a message quoting its MAC code. In a similar way, MAC codes can be used to control which machines on a network can access files and other shared resources. For example, I've set up my wireless link to the Internet so that only two MAC codes can ever gain access to it (restricting access to the network cards built into my two computers). That helps to stop other people in nearby buildings (or in the street) hacking into my connection or using it by mistake.

The bigger you make a network, the more extra parts you need to add to make it function efficiently. Signals can travel only so far down cables or over wireless links so, if you want to make a big network, you have to add in devices called repeaters—effectively signal boosters. You might also need bridges, switches, and routers—devices that help to link together networks (or the parts of networks, which are known as segments), regulate the traffic between them, and forward traffic from one part of a network to another part.