Right.
Whenever your router device connects to your ISP it can get a different, new IP address.
All that means (if people aren't trying to join through the GameSpy Halo Lobby...) is that you need to check that IP address every tome before you start asking people to join your server. (Don't worry, tho! It won't change WHILE YOU'RE PLAYING in the server!)
And actually, its good for your computer's security for that (WAN) IP to change from time to time....