No, its not so straightforward as that! The licence plate analogy is a good one. Your neighbour's licence plate may be similar to yours, but there's no need for it to be. As a rule, the higher parts are owned by an organisation which then re-allocates the lower parts to other people or organisations and so on, so AOL people have similar numbers (maybe the top part the same), NTL people have other similar numbers, and so on. So if your neighbour is NTL but you are AOL, the numbers will be quite different.

Of course, its more complicated than that, especially if the number is, say, 10.*.*.*