Anyway, back to packet switching. When you send a file over the Internet, you don’t actually send the whole thing in one big chunk. It's broken up into small packets -- like postcards if you want to continue the post office simile -- and each one is transmitted one after the other. The beauty of this is that the routers can handle many, many of these little packets, without ever having to know what's in them (or indeed, the order they are transmitted in). So your packets get mixed in with someone else's, and the data stream gets maximum efficiency. All your machine has to do is create the little packets, number them so they get re-assembled on the other end in the correct order, and send them out to the router. Of course they need an address too.

That's where IPs come in. An IP is a unique address for your machine on the Internet. It’s a 4-digit number, all of which are between 0 and 255. For instance 204.57.198.32. All those www.whatever.com names are actually converted into IP addresses when packets are exchanged with another machine on the net. Sometimes these are specific and constant on one machine, more often than not they are dynamically allocated by the host system. Every time you log onto you service provider, they send you an IP address they have free from a range that's been allocated to them. For instance, your ISP may have the range 204.198.32. 0 to 255, which gives them 256 possible IP addresses. 256 people can all be using the system at once, but no more than that. When you log in, the system looks to see what IP's are free, and sends you one. That way more than 256 people can be on the books for this Host, but only 256 can use it at once.

The alternative to this would be the phone system approach, which would mean creating dedicated routers that would reserve an entire line for you to send data to and from the other computer, but that would not get used most of the time, especially if you are doing stuff like typing in real time. You may think you are a fast typist, but in the time between a message going to and from your machine to another, the network could have transmitted War and Peace several times. A good simile that I heard used is, “like reserving the entire Interstate road system to drive a car from Washington DC to LA.” You would never dream of doing that, instead you share it with other car drivers. Just like on the Internet. Maybe that dumb 'super highway' label thing has some merit after allJ

I'm sure you can see how sharing lines with others and breaking messages into small packets is the most efficient use of network time and data streams. The same system is in use today as was originally designed for the ARPANET way back when. Why? ´Cos it works real well. :)

Where do ISPs come into this? Lets think of it this way. The routers are machines that sit attached to mainframes and stuff that we are treating as big post offices. An ISP (Internet Service Provider) is one step removed from that - like the postman himself. They are attached to a machine that often has a router (not always), but they also have a ton of modems attached to them. Your little PC at home uses its modem to call up the modem attached to the ISP's machine, which then accepts your packets and forwards them in bulk and mixed in with everyone else's, to the Internet with a capital I.

Cable modems, ISDN and DSL are pretty much the same thing, except that the modem-to-modem part is removed, and faster bandwidth communication devices are used instead. In fact DSL is basically just a faster modem with a better phone line anyway. :)