To understand what a router does, you need to know a little bit about how the Internet works. The Internet, the huge world-wide computer network that we surf the World Wide Web on, uses the TCP/IP networking protocol. Data sent over a TCP/IP network is broken down into chunks called "packets", and in order for these packets to get where they need to go on the Internet, something is needed to route them to their destinations (hence the name router). The many networks that make up the Internet are connected to each other by routers, and the routers determine what packets go where. Routers are level 3 devices on the OSI model, which means that they function at the network layer. The OSI model has 7 levels, and each one represents a function when it comes to data communication between two or more computers on a computer network. To understand what I'm talking about in the next paragraph, you need to know about the OSI model and what layer the router represents. When the average computer user thinks of a router, they probably think of the kind that you can purchase at your local computer store, as opposed to the big ones that route packets on the Internet. That smaller kind of router allows you to share an Internet connection with multiple computers without having to use a computer that acts as a "gateway". If you have a computer network at home and the center of it is a hub or a switch, you need to use a computer that acts as a gateway ("host" computer), as hubs and switches cannot do what a router does (hubs and switches are two other devices that you can purchase at your local computer store to create a network between two or more computers). A router can "intelligently" route TCP/IP packets to IP addresses (unique identifiers on a TCP/IP network) at layer 3, while a hub is just a dumb layer 1 device that sends out packets to all of the computers attached to it, regardless of who sent them out and who they're intended for. A switch is a layer 2 device that routes Ethernet frames by MAC address; Ethernet is a networking standard that home networks are based on, a frame is the Ethernet equivalent of a TCP/IP packet (chunk of data), and a MAC address is a computer's unique identifier at layer 2 (datalink) on an Ethernet network. Although a switch is "smarter" than a hub, it still can't do what a router does. The functions that a router performs are necessary for communication between the computers on your network and computers on other networks on the Internet at the network layer (TCP/IP). Probably the most important thing that a router does is perform NAT, which stands for Network Address Translation. NAT is a technology that allows computers on a network behind an Internet connection to communicate with computers outside the network; to the rest of the Internet the network is a single IP address (the one your ISP gave you), and NAT allows for incoming and outgoing connections between the computers on the network who have their own "internal" IP addresses and the computers outside the network who see only the ISP-assigned IP address. A "gateway" computer using Internet connection sharing software like the kind that Windows comes with performs the same functions as a hardware router, but there are some disadvantages that come with using that setup, like how all of the computers on the network lose Internet access if the gateway computer is offline. If you want to share an Internet connection between two or more computers in your home, a hardware router is the best thing to get for it.

Routers are specialized computers that send your messages and those of every other Internet user speeding to their destinations along thousands of pathways. In this article, we'll look at how these behind-the-scenes machines make the Internet work. Keeping the Messages Moving: When you send e-mail to a friend on the other side of the country, how does the message know to end up on your friend's computer, rather than on one of the millions of other computers in the world? Much of the work to get a message from one computer to another is done by routers, because they're the crucial devices that let messages flow between networks, rather than within networks. Let's look at what a very simple router might do. Imagine a small company that makes animated 3-D graphics for local television stations. There are 10 employees of the company, each with a computer. Four of the employees are animators, while the rest are in sales, accounting and management. The animators will need to send lots of very large files back and forth to one another as they work on projects. To do this, they'll use a network. When one animator sends a file to another, the very large file will use up most of the network's capacity, making the network run very slowly for other users. One of the reasons that a single intensive user can affect the entire network stems from the way that Ethernet works. Each information packet sent from a computer is seen by all the other computers on the local network. Each computer then examines the packet and decides whether it was meant for its address. This keeps the basic plan of the network simple, but has performance consequences as the size of the network or level of network activity increases. To keep the animators' work from interfering with that of the folks in the front office, the company sets up two separate networks, one for the animators and one for the rest of the company. A router links the two networks and connects both networks to the Internet. Directing Traffic The router is the only device that sees every message sent by any computer on either of the company's networks. When the animator in our example sends a huge file to another animator, the router looks at the recipient's address and keeps the traffic on the animator's network. When an animator, on the other hand, sends a message to the bookkeeper asking about an expense-account check, then the router sees the recipient's address and forwards the message between the two networks. One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including: Information on which connections lead to particular groups of addresses Priorities for connections to be used Rules for handling both routine and special cases of traffic A configuration table can be as simple as a half-dozen lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages. A router, then, has two separate but related jobs: The router ensures that information doesn't go where it's not needed. This is crucial for keeping large volumes of data from clogging the connections of "innocent bystanders." The router makes sure that information does make it to the intended destination. In performing these two jobs, a router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. As the number of networks attached to one another grows, the configuration table for handling traffic among them grows, and the processing power of the router is increased. Regardless of how many networks are attached, though, the basic operation and function of the router remains the same. Since the Internet is one huge network made up of tens of thousands of smaller networks, its use of routers is an absolute necessity. Transmitting Packets When you make a telephone call to someone on the other side of the country, the telephone system establishes a stable circuit between your telephone and the telephone you're calling. The circuit might involve a half dozen or more steps through copper cables, switches, fiber optics, microwaves and satellites, but those steps are established and remain constant for the duration of the call. This circuit approach means that the quality of the line between you and the person you're calling is consistent throughout the call, but a problem with any portion of the circuit -- maybe a tree falls across one of the lines used, or there's a power problem with a switch -- brings your call to an early and abrupt end. When you send an e-mail message with an attachment to the other side of the country, a very different process is used. Internet data, whether in the form of a Web page, a downloaded file or an e-mail message, travels over a system known as a packet-switching network. In this system, the data in a message or file is broken up into packages about 1,500 bytes long. Each of these packages gets a wrapper that includes information on the sender's address, the receiver's address, the package's place in the entire message, and how the receiving computer can be sure that the package arrived intact. Each data package, called a packet, is then sent off to its destination via the best available route -- a route that might be taken by all the other packets in the message or by none of the other packets in the message. This might seem very complicated compared to the circuit approach used by the telephone system, but in a network designed for data there are two huge advantages to the packet-switching plan. The network can balance the load across various pieces of equipment on a millisecond-by-millisecond basis. If there is a problem with one piece of equipment in the network while a message is being transferred, packets can be routed around the problem, ensuring the delivery of the entire message. The Path of a Packet The routers that make up the main part of the Internet can reconfigure the paths that packets take because they look at the information surrounding the data packet, and they tell each other about line conditions, such as delays in receiving and sending data and traffic on various pieces of the network. Not all routers do so many jobs, however. Routers come in different sizes. For example: If you have enabled Internet connection sharing between two Windows 98-based computers, you're using one of the computers (the computer with the Internet connection) as a simple router. In this instance, the router does so little -- simply looking at data to see whether it's intended for one computer or the other -- that it can operate in the background of the system without significantly affecting the other programs you might be running. Slightly larger routers, the sort used to connect a small office network to the Internet, will do a bit more. These routers frequently enforce rules concerning security for the office network (trying to secure the network from certain attacks). They handle enough traffic that they're generally stand-alone devices rather than software running on a server. The largest routers, those used to handle data at the major traffic points on the Internet, handle millions of data packets every second and work to configure the network most efficiently. These routers are large stand-alone systems that have far more in common with supercomputers than with your office server.