Apple's [http://www.apple.com/airportexpress/ Airport Express] has and [http://www.apple.com/airportextreme/ Airport Extreme] have some funky settings that make it hard to use with other networking devices, including your Amahi server.
There is no option to disable the DHCP in the Airport admin tool, so what you need to do is trick it into being "more pasive"
* Have a computer request an address while your Amahi HDA is offline
* After that, put Amahi back online
* Disconnect the Airport Express/Extreme
* Have that same computer request an address
* After that you can turn on the Airport Express/Extreme
Look at a more [http://discussions.apple.com/thread.jspa?threadID=121990 detailed explanation].