Settings for Firewall and AirPort Extreme Router to Open a Port
Something's wrong.  If you've only got the Apple device between yourself and the internet - you've verified your external IP, you've lowered your machine's firewall, and you've DMZ'd your internal IP (which you've verified is correct...) - then the only way a connection can be halted is through another piece of software running on your machine.

If you go through all of those steps, you should be completely open to the internet.