Here’s a great tip for anyone trying to use the ‘extend a wireless network’ option using an Airport Express, what should have been a simple standard Apple like experience has previously left me without a working network and randomly unplugging cables before.
The reason is that Apple thinks that extending a network should be done wirelessly, and that if you have your Airport Express plugged into your network, it will send out all kinds of strange calls that will effectively knock you offline (I believe its something to do with both boxes running DHCP, the service to allocate IP addresses to clients).
So you’ve got an easy choice, unplug the ethernet and all will work again. However, if you’re anything like me, you already have your house wired up with far superior ethernet, and want to simply extend the network into a different location over cat5 rather than rely on the fragile Wifi signals. The answer is surprisingly simple, just create a new network, but make sure it is using the exact same settings and passwords as the original, that way clients will happily pick the basestation with the stronger signal, and avoid having to configure all your devices for another network.
Finally you will want to drop into the Network menu in Airport Utility and configure the router mode to ‘Off’ and don’t forget to setup a 5ghz wireless network under ‘wireless’ and ‘wireless options’. This does create other network, but the speed advantages are substancial and can help cut through areas where the are lots of 2.4ghz networks operating (but not all hardware can support it so it’s good to use both).
via Apple Support Communities.