Another good alternative is
http://ws.arin.net/whois/?queryinputI deal with a lot of users on my sites, and use IP address checks to see if there are multiple users as well.
In the case of AOL I have seen IP addresses change as fast as I can refresh my pages

Think they run on some kind of proxy system.
Generally, when I get AOL IP addresses, I look at other account specific features. Amazing how many poeple signing up for multiple accounts, will claim to be individuals but they then use the same email address or password on signup.
Even had a few use the password of one account, as the username of the next account
