Whatismyip.org shows you your external IP address. Unless your computer is connected directly to the internet (i.e. no router), any code you run that only looks at the local machine can only give your your internal address.

If you want to be sure to always get the external IP, you will have to use an external service like whatismyip.

As for changing that IP address, that is very dependent on the internet connection that is being used and how the IP address is obtained.