To ensure that requests are properly routed to your web, AppleShare, or FTP server, or a specific computer on your network, you need to establish a permanent IP address for the server or computer, and provide “inbound port mapping” information to the AirPort Base Station or Time Capsule.
Open AirPort Utility, located in the Utilities folder in the Applications folder on a computer using Mac OS X, and in Start > All Programs > AirPort on a Windows computer.
Select the device you’re setting up, and then click Manual Setup. Enter the password if necessary.
Click Advanced in the toolbar, and then click Port Mapping.
Click the Add (+) button and choose a service from the Service pop-up menu (for example, Personal File Sharing).
Enter the IP address of the server in the Private IP Address field.
To use port mapping, you must configure TCP/IP manually on the computer that is running the web, AppleShare, or FTP server.