Here are the two queries that you would need. I've just been executing these from the SQL Command
option in the Admin Control Panel:
SELECT DISTINCT B_IP
Replace <username> with the username of the poster you're investigating. This will show you all of the IP addresses used by an individual poster. Make sure it's spelled exactly the same.
Replace XXX.XXX.XXX.XXX with the appropriate IP address. This is good for seeing if two usernames are using the same IP address.