IP address is not a reliable way of tracking logins. For example I have 4 computers through a single router. This means all 4 have the same exact IP address (you can't see my local). So if 3 of the 4 login then you'll see 3 logins from the same IP address. This is majorly prominent if a user is logging in from an internet cafe, hotel internet, school campus, etc.. In addition to this I have a dynamic IP like most of the world so more or less on a weekly basis my IP changes (for all 4 computers). So if you track a single IP address all they need to do is reboot their modem and they've new address.
At any rate the best way to do this is develop an CB plugin to act on the login trigger then store the IP to database for a user. More on trigger usage in tutorials within my signature.