Before I installed the Airport Express, I used my DLink 504T broadband gateway to provide NAT and DHCP services for the network (including the Airport Extreme). I want to continue with this setup, since it lets me share itunes between my wired computers and my wireless ones -- but the Assistant doesn't like this configuration. However, it is possible to configure the airport express without changing the DHCP configuration of your network.
First, select your Airport Extreme base station using the Wireless icon in the top right hand corner of your desktop. Open the Airport Admin Utility, and select your Airport Extreme. Note down the Mac address at this point. Now click Configure, and:
- Take a backup of the configuration: File -> Save A Copy As
- take a note of the channel it transmits on.
- In Internet, check that it's configured using DHCP. If it isn't, stop reading this document now -- it's not for you
- Change the Security to either none or WEP (see note below).
- In the WDS tab, select Enable this Base Station as a WDS main base station, and Allow wireless clients. Don't select any remote Base Stations at this point.
- update the base station
- Enter a better password
- Give your Base Station a name
- Choose a network name (SSID) - it can be anything
- Select the same channel as the Airport Extreme
- In Internet, Configure using DHCP
- In Network, do not Distribute IP addresses.
- In WDS:
- Enable this base station as a remote base station
- Allow wireless clients on this base station
- Enter the MAC address of your Airport Extreme (you took a note of it before
- press Update and wait for it to come back online
Caution: I haven't tested these exact instructions, sorry -- but I got mine working!
Note 1 -- Hard reset: unplug your airport express and hold down the reset button while plugging it back in. Keep the button held down until the light flashes green 4 times, then unplug and re-plug the extreme. Wait 1 minute for it to start.
Note 2 -- Using WDS at all prevents you from using the more secure WPA security on your wireless networking. If you follow these instructions, you are doing so at your own risk -- don't blame me if someone hacks your WEP network!

