You mean for using your account?
If it's a dial-up connection, most probably the IP is dynamic, so it's useless.
If it's cable connection, then not only the IP identifies your computer, but the modem's MAC-address too. So it won't work.
You cant "steal" it because its most often randomly given out (and if you get that attatched to the number ... well......) now they can spoof your ip but thats not really stealing since you will still have your ip
