Set a default host computer
You can designate a default host computer on your network that is exposed to the Internet and receives all inbound traffic. A default host may be useful if you use a computer on your AirPort network to play network games, or want to open up any incoming Internet traffic to a single computer.
Open AirPort Utility, located in the Utilities folder in the Applications folder.
In the graphical overview, select the base station you want to configure, then click Edit. You may need to enter a password for the base station.
Click Network, then verify that “DHCP and NAT” is selected in the Router Mode pop-up menu. A default host can only be configured when the base station is acting as a NAT router.
Click the Network Options button near the bottom of the pane. In the dialog that appears, select “Enable default host at,” then enter the IP address of the computer you want to set up as the default host.
Important: During setup, AirPort Utility Setup Assistant automatically determined whether “Enable IGMP Snooping,” “Block incoming IPv6 connections,” “Allow Teredo tunnels,” and “Allow incoming IPSec authentication” should be turned on or off. Don’t modify these settings unless you’re instructed to do so by your ISP or network administrator.
Click Save to close the dialog, then click Update to save the changes.