Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Which one?

The one your Mac has internally while connected to the internet, or the one your ISP gave you?

Internal:

ethernet.jpg

The same can be used for Airport.


External:
http://whatismyipaddress.com/
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.