The first and most basic job of the router is to know where to send information addressed to your computer. Just as the mail handler on the other side of the country knows enough to keep a birthday card coming toward you without knowing where your house is, most of the routers that forward an e-mail message to you don't know your computer's MAC address, but they know enough to keep the message flowing.
Routers are programmed to understand the most common network protocols. That means they know the format of the addresses, how many bytes are in the basic package of data sent out over the network, and how to make sure all the packages reach their destination and get reassembled. For the routers that are part of the Internet's main "backbone," this means looking at, and moving on, millions of information packages every second. And simply moving the package along to its destination isn't all that a router will do. It's just as important, in today's computerized world, that they keep the message flowing by the best possible route.
In a modern network, every e-mail message is broken up into small pieces. The pieces are sent individually and reassembled when they're received at their final destination. Because the individual pieces of information are called packets and each packet can be sent along a different path, like a train going through a set of switches, this kind of network is called a packet-switched network. It means that you don't have to build a dedicated network between you and your friend on the other side of the country. Your e-mail flows over any one of thousands of different routes to get from one computer to the other.
Depending on the time of day and day of the week, some parts of the huge public packet-switched network may be busier than others. When this happens, the routers that make up this system will communicate with one another so that traffic not bound for the crowded area can be sent by less congested network routes. This lets the network function at full capacity without excessively burdening already-busy areas. You can see, though, how Denial of Service attacks (described in the next section), in which people send millions and millions of messages to a particular server, will affect that server and the routers forwarding message to it. As the messages pile up and pieces of the network become congested, more and more routers send out the message that they're busy, and the entire network with all its users can be affected.