Configure Airport to routers DMZ IP

As Rob says, "Default Host" is what you are looking for. The current version (as of June 2015) of the Airport Utilty App (iOS) uses the following path:

1. Open Airport Utility
2. Select the Airport
3. Click 'Edit'
4. Click 'Advanced'
5. Click 'DHCP and NAT'
6. Click 'Default Host', and enter the IP address you want.

And the Airport Utility App (Mac) uses this path:

1. Open Airport Utility
2. Select the Airport
3. Click 'Edit'
4. Click 'Network'
5. Click 'Network Options'
6. Click 'Default Host', and enter the IP address you want.

Separately, you would probably also go into 'Reservations' on the DHCP and NAT page so that you can map one certain device's MAC address to the IP you just set.

For example, to get XBoxLive to play nice, find the MAC address for your XBox (from the XBox menus), then set a reservation from that MAC address to a specific IP address that you choose, then set the 'Default Host' to that same IP. That will put your XBox on the DMZ.

Apple uses the wording "Default Host" for a DMZ.

1. Open AirPort Utility
2. Select the AirPort
3. Click Edit
4. Click the Network tab
5. Network Options
6. Click "Enable default host at"
7. enter the local IP you wish to use and done.

I hope you have the same version and same options visible as I do...