Instructions
Login to your router's configuration interface. This is usually done by entering the router's internal IP address (usually "192.168.0.1," "192.168.1.1," "192.168.2.1" or something similar) into the address bar of a Web browser.
Click on the "WAN Setup" or similarly named link in the "Advanced" section of the router menu's navigation pane.
Select "Open," "Unsecured" or "Unrestricted" (which option depends on the router manufacturer) next to "NAT Filtering." Click the "Apply" or similarly named button to save your settings. Your NAT is now unrestricted and you will have full access to the multiplayer on the "Halo 3" video game.