AOL are pretty annoying in terms of IP etc. I encountered the same thing on a board I was running some time ago - lots of users apparently on the same IP - all AOL users.
"The problem is some ISPs don't arrange their IP allocation particularly well, either for historical or other reasons ... AOL is one significant culprit."
^^^ an excerpt from an article on
www.phpbb.com
(
http://www.phpbb.com/kb/article.php?article_id=54 )