You can configure the AirPort Extreme to "extend a wireless network" and in this case, the Ethernet ports are enabled.
Only the 802.11n versions of the AirPort Express have the ability to "join" a wireless network and provide an Ethernet signal at the port. This is an undocumented feature known as ProxySTA.
The Airport Express is far too bandwidth limited for my needs (having only fast Ethernet ports). I need the Airport device to support a Mac Pro (with dual gigabit ethernet) and a Netgear NAS (with gigabit jumbo frame Ethernet), my backbone network is gigabit with jumbo frame support. If you are correct about not being able to configure the Extreme as a bridge then I guess I'm stuck with running the Extreme in extend mode. I'm mainly concerned about wireless network reliability with the RF of both Airport devices going, I've had problems with network reliability with this type of setup in the past. I was also trying to limit the total RF, my location is a Condo complex with many nearby having their own WiFi setups.
If anyone knows of an 11n bridge available with gigabit ports (minimum 2, 3 ideal), please let me know. Perhaps I should consider getting an Airport adapter for my MacPro2,1.
If you are correct about not being able to configure the Extreme as a bridge then I guess I'm stuck with running the Extreme in extend mode.
We only answer questions when we've tested it ourselves. But, you can prove to yourself that the AirPort Extreme cannot act as a simple wireless to Ethernet bridge by trying to configure it to "join" a wireless network. You will see that the Ethernet ports are not enabled in this type of setup.
Configuring the AirPort Extreme to "extend" a wireless network will enable the Ethernet ports. I'm sure that you understand that although the Extreme has Gigabit Ethernet ports, the "Ethernet" signal that appears at the LAN ports will only be as fast as the wireless connection to the AirPort Extreme allows.
Perhaps surprisingly, the best that Cisco has to offer in terms of a wireless to Ethernet bridge is an 802.11b/g device with 100 Mbps Ethernet ports. Netgear does a bit better at least offering 802.11n capability, but still 100 Mbps ports.
The AirPort adapter or a USB wireless adapter might be the simplest way to go if you are concerned about adding an additional wireless access point. Good luck.
I was able to find three vendors that have devices that work as a 11n bridge and have gigabit Ethernet ports. These devices are marketed as high bandwidth streaming bridges for media content, the term "media bridge" appears to be common. For reference I am listing them here.
- TRENDnet HD Media Bridge TEW-680MB (four ethernet ports)
- Buffalo Airstation WLI-H4-D1300 (four ethernet ports, supports 11ac)
- D-Link DAP-1525 (four ethernet ports, configurable bridge/AP functionality)
Apple may want to consider positioning the Airport Extreme to be configurable similarly to these or release a new version of the Airport Express with gigabit ports, clearly there is a market for this type of device.
Will post status update when I decide what to do.