Open AirPort Utility, select your base station, then click Edit.

Open AirPort Utility for me

Click Network, then choose DHCP and NAT from the Router Mode pop-up menu.

Click Network Options, turn on Enable NAT Port Mapping Protocol, then click Save.

Click Update to restart the base station with the new setting.
Note:   The wireless network and all its services are unavailable until the base station restarts completely.