The user IPs are captured when the user logs in (clicks the login link), and once per day on the first request of the day. The new IP takes the place of the previous IP.
All of these IPs are then indexed in a new file called act_index, along with the registration IP and last post IP.
While there's no UI to search all of the IPs at once, you can search a particular category of IP in the control panel.
In addition, the user post history file also stores all of the IPs for all of the user's posts. While there is no UI to search for IPs for these history files, they can be found in the Members/user_posts/ directory, named after the user numbers. I think that the IPs might be present in the 6.3 that you're using, but I don't recall. In any case, as of 6.6 (and thus 6.7), the admin can get a snapshot view of all the user's unique IP addresses by viewing the user post history in the control panel.
