Open Airport Utility, and once it finds your base station, open it and click on the advanced tab. Under advanced, click on port mapping. Under port mapping click the '+' sign to add a new map. In the resulting pop up window, leave select a service as is. Then put in the ports you need and the IP of the machine you are mapping to. Click continue, and in the next screen give it a name and then click done. It will then put you back in the mapped list screen and you'll need to click update for the changes to be saved to the base station. That should do it!
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