What's This 'Internet' I Keep Hearing About? - Part Five - Making Contact

I've established how the Internet is a computer network: now I will explain how two computers that could be on other sides of the world can send messages to each other.

Imagine you were writing a letter and needed to send it to someone. If you just wrote a name on the front, it would never arrive, unless perhaps you lived in a small village. A name is rarely specific enough. Therefore, as we all know, we use addresses to contact someone, often using: the name, the house number, the road name, the town name, the county name, and sometimes, the country name. This allows sending of messages on another kind of network - the postal network. When you send a letter, typically it will be passed between postal sorting offices starting from the sorting office nearest to the origin, then up to increasingly large sorting offices until it's handled by a sorting office covering regions for both the origin and the destination, then down to increasingly small sorting offices until it's at the sorting office nearest the destination - and then it's delivered.

In our postal situation, there are two key factors at work - a form of addressing that 'homes in' on the destination location, and a form of message delivery that 'broadens out' then 'narrows in'. Computers are more organised, but they actually effectively do exactly the same thing.

Each computer on the Internet is given an address ('IP address'), and this 'homes in' on their location. The 'homing in' isn't done strictly geographically, rather in terms of the connection-relationship between the smaller computer networks within the Internet. For the real world, being a neighbour is geographical, but on a computer network, being a neighbour is having a direct network connection.

Like the postal network with its sorting offices, computer networks usually have connections to a few other computer networks. A computer network will send the message to a larger network (a network that is more likely to recognise at least some part of the address). This process of 'broadening out' continues until the message is being handled by a network that is 'over' the destination, and then the 'narrowing in' process will occur.

An example 'IP address' is '69.60.115.116'. They are just series of digit groups where the digit groups towards the right are increasingly local. Each digit group is a number between 0 and 255. This is just an approximation, but you could think of this address meaning:

• A computer 116
• in a small neighbourhood 115
• in a larger neighbourhood 60
• controlled by an ISP 69
• (on the Internet)

The neighbourhoods, the ISP, and the Internet, could all be consider computer networks in their own right. Therefore, for a message to the same 'larger neighbourhood', the message would be passed up towards one of those intermediary computers in the larger neighbourhood and then back down to the correct smaller neighbourhood, and then to the correct computer.