Step 3: DMZ for You and Me
To access a Mac on a home network that uses a router, you need to designate that Mac as the DMZ (demilitarized zone)-or Default Host in AirPort Extreme parlance. Launch the AirPort Admin Utility (/Applications/Utilities), log into your Base Station, and click the AirPort tab. Click the Base Station Options button, check the box labeled Enable Default Host At, keep the default setting (10.0.1.253), and click OK. If you're using a dynamic-IP account, click the Internet tab, check the Always Stay Connected box, and click Update. After the Base Station refreshes, note the IP address reported in the Configure TCP/IP section-that's the server address you'll use to connect from afar.
Once you enable a default host or DMZ, the router's firewall will let you right in.
