Solution
An AirPort Base station treats Ethernet and AirPort as two disconnected network segments. Turning on the "AirPort to Ethernet bridging" option in the AirPort Admin Utility allows you to connect those two segments for services such as File Sharing. For detailed instructions and information on designing an AirPort network, review the document "Designing AirPort Networks" to see what works best for you. You may find the document at Apple Software Updates (http://www.apple.com/swupdates/).
If you have a cable modem and need to bridge Ethernet and AirPort, see this article:
Article 106021: "AirPort: How to Set Up Bridging When Using a Cable Modem"
Important: Bridging may adversely affect other network services in certain environments. If you have a cable modem, you should not use the AirPort Base Station for bridging combined with NAT (network address translation) and DHCP services. NAT and DHCP services may be run on a separate router.
Note: Computers that are set up as AirPort software base stations do not have a bridging feature.
Note: Wireless Internet access requires an AirPort Card, AirPort Base Station, and an Internet service provider, for which fees may apply. Some ISPs are not compatible with AirPort. America Online (AOL) works with AirPort software version 2.0 or later. For more information on AOL compatibility, see technical document 106591: "AirPort: How to Use with AOL." Range may vary with site conditions.
Sharing with AirPort Base Station (Dual Ethernet) or AirPort Extreme Base Station
Bridging from wireless AirPort clients to the LAN port of the AirPort Base Station (Dual Ethernet) and AirPort Extreme Base Station is automatically enabled. You do not need to follow these steps. You may share across the WAN port when you are not using network address translation (NAT) to share a single IP address.