Eh, I may be wrong: The class of IP address depends on how many subaddresses are available:

Here's some more tech-guru-ish info. Basically, I blocked every address within several smaller networks, within a Class A network...this was done because the evil user logged in via a dialup connection, which means every time he dials up he gets a different IP address - the Class D address will always be different, and if he dialed a different POP number, he'd probably get a new Class C address, but the Class B address of his ISP is unlikely to change, so that's what I blocked. It also means that everyone else dialing up will be blocked as well, but, oh well, it can't be helped.

I may be wrong, too -- I'm not a network guru...