To really show the problem, you have to do something like contrast how "blogspot.com" is a top-level site, one level below a TLD, but so is bbc.co.uk, one level below what "co.uk". The naive "count one element" doesn't work, or all of "co.uk" would share cookies. And it turns out that now there just isn't much you can do other than have a huge table. Sure, we'd probably do it differently if we had it to do all over again, but, we don't.