IP address and wp database

Hi guys. I’m looking for a little help with something I’m wanting to do.
Obviously you know that when a user leaves a comment their IP address
is stored in the WP database. I want to be able to echo “Welcome back $user” (ip address being matched to the $user) the only thing is I don’t know how to check the IP address from the person visiting against the IP addresses that are in the database. I am a newb with very little experience with php and sql queries. Any help is much appreciated guys. Thanks.