Airport express

Apple's Airport Express and Airport Extreme have some 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"

• Set the pool of addresses to only one address
• Set the DHCP lease length to 8000 days or something long
• 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 detailed explanation.

BTW, it's much easier if you don't want NAT (i.e. if you have another router that does that), then you can disable both NAT and DHCP by choosing bridge mode. You just can't disable DHCP alone.