Apple Airport - Access DHCP from ethernet connection?
The DHCP works fine for the airport.

If you use ethernet and a static IP address, then you'll have to choose an IP address outside of the DHCP lease range otherwise you can get IP address conflicts.

For example, if your DHCP range is set to 192.168.0.10 through 192.168.0.20 then you can use the IP address ranges 192.168.0.1 - 192.168.0.9 and 192.168.0.21 - 192.168.0.253 for statically assigned IP address.

Hope this helps