I always just assumed the TinyURL people were generating URLs, as the requests came in, in some base where you don't have to add a column any time too soon, as in, the first 16 million TinyURLs are in the space 000000 .. ffffff and when you run out you move to the 00000000 .. ffffffff space, which should keep you going for a while. I mean, they're not actual numbers, so 00000000 isn't the same as 000000.