May be IP address, but that's kind of a flawed method considering dialup per-session dynamic IP addressing, hence possibility for collateral denial. I'm thinking it's probably a cookie within your browser software; most modern cart software is anyway. Try clearing the cookies within your browser, and then restarting the browser and ordering again. There are many ways to do this depending on what your browser is, if it's Mozilla I can help, if IE someone else will know. Give that a shot then holler back..... If that doesn't work it could be IP address AND cookie, or just the IP address alone as others stated, we can usually get a new IP address unless the carrier doesn't allow your hardware address to age out or be replaced without calling. Clear cookies first and that will work in many cases...
HTH
J