The Internet

So, all of our computers are networked together. What can we do next?

We're not alone. As well as our network looking after computers in our local area (a local area network, or LAN), there are loads of others out there, so the next obvious step is to connect them all together into one big inter-network system (a wide area network, or WAN).

Yet again, we do this by stealing ideas from the telephone service. As mentioned earlier, there are thousands of telephone exchanges, just like there are thousands (or more likely, millions) of LANs. How did the telephone companies manage to identify all of the phones across all of the exchanges?

They extended their local numbering system by adding area codes. If the number you dial contains a non-local area code, then instead of connecting your phone to another local one, you are connected to a different exchange (usually one who looks after that specific area code, but sometimes you might pass through several exchanges before you arrive at the final one - each one in between getting you slightly closer) and they then connect you to your destination.

So, our network does the same. If we know that we want to connect to a computer outside our local network, rather than making a direct connection to another local computer we make one to a device that behaves a little like a telephone exchange which connects our network to one or more others.

How does our device know which remote network to connect to?

It has the computer equivalent of a road map and will look up our destination computer and plan a route between our local network and the one we need to connect to. Once it knows the route, it sends our data on to the next connection. Like the telehone exchange, that connection might be the final one, or might be just another part of a chain of connections, each closer to your destination than the last. Given its job, you won't be entirely surprised to learn that this device is called a router.