This is from Apple documentation regarding their version of port forwarding.
AirPort Extreme uses Network Address Translation (NAT) to share a single IP address with the computers that join the AirPort Extreme network. To provide Internet access to several computers with one IP address, NAT assigns private IP addresses to each computer on the AirPort Extreme network, and then matches these addresses with port numbers. The wireless device creates a port-to-private IP address table entry when a computer on your AirPort (private) network sends a request for information to the Internet.
If you are using a web, AppleShare, or FTP server on your AirPort Extreme network, other computers initiate communication with your server. Because the Apple wireless device has no table entries for these requests, it has no way of directing the information to the appropriate computer on your AirPort network.
To ensure that requests are properly routed to your web, AppleShare, or FTP server, you need to establish a permanent IP address for your server and provide inbound port mapping information to your Apple wireless device.
To set up inbound port mapping:
1. Open AirPort Utility, select your wireless device, and then choose Manual Setup from the Base Station menu, or double-click the device icon to open its configuration in a separate window. Enter the password if necessary.
2. Click the Advanced button, and then click Port Mapping.
3. Click the Add ➕ button and choose a service, such as Personal File Sharing, from the Service pop-up menu.
Type any additional information you need in the text fields.
To use port mapping, you must configure TCP/IP manually on the computer that is
running the web, AppleShare, or FTP server. You can also set up a computer as a default host to establish a permanent IP address for the computer and provide inbound port mapping information to the AirPort Extreme Base Station or AirPort Express. This is sometimes known as a DMZ and is useful when playing some network games or videoconferencing.
To set up a default host:
1. Open AirPort Utility, select your wireless device, and then choose Manual Setup from the Base Station menu, or double-click the device icon to open its configuration in a separate window. Enter the password if necessary.
2. Click the Internet button, and then click NAT.
3. Select the “Enable Default Host at” checkbox. The default IP address is 10.0.1.253.
4. Enter the same IP address on the host computer.
3389 is the port needed for Windows Remote Desktop.
