Yeah, there are programs which can give you the IP just by using the user #, but seriously, if you have a firewall you really don't have to worry, unless you have your entire drive shared for no reason, have no firewall, don't use a password for file sharing, or have trojans on your computer. Besides, there are at least 10 million users (assuming user #s are assigned in numerical order and most people don't multiples), and the chances of you being even targeted for an attack is slim.