Or to troll. A while ago, skype had a vulnerability that let people resolve skype names to IP addresses and since skype is the default communication of lazy gamers, it was pretty trivial to work out the correct IP address to poke. Other methods used to include hooking your Xbox360 up to your laptop and bridging your connection, running Cain and Abel and finding the addresses of opponents that way in games like Halo etc. No idea what the scene is like now that we're on xbones and ps4s, I pretty much exclusively game on PC now. There are plenty of services online that let you snatch IP addresses by sending a link to a 'friend', you could then keep tabs on them that way.