Short Answer:
Another way to do it is to ask the internet.
There are many sites but the simplest is probably
http://WhatIsMyIP.org/
Longer Answer:
Many people have LANs now. Local Area Networks. Their computer talks to their router, their iPod, their laptop, their Xbox, etc. People tend to confuse the addresses used "in house" that the different toys use to talk to each other, with those that the internet sees as the address for the entire house. Using an internal LAN IP number is like yelling at people on the net "Im in the bedroom!". They need the house address.
If you visit a site such as I posted then it is telling you the address it sees you as coming from. The address that the net sees your entire LAN. That is the address that people out on the net will have to use to get to your machine.