Airport Networking Issue


I think this is a networking problem.

I was happily sharing a cable internet connection from an airport express to two macs - iMac running osx10.4 and powerbook running tiger.

I bought a new airport extreme basestation (to extend the range).

Plugged it in, configured it as the main base station, configured the express as a remote base station.

Powerbook connected straightaway to the network and to the internet. iMac says it is connected to network and internet but any attempt to connect to internet fails. Also I can no longer see shared local files on iMac.

All configured to DHCP and basically everything is connected to automatic.

I thought it may have been an incompatibility and (I was about to do this anyway) so I upgraded the OS on the iMac to Tiger. No change.

I know there are a lot of variables but any clues on where to start would be appreciated....
Is the iMac connecting to the main base station or to the Airport Express repeater?

Have you tried it on the other way around? Eg: If the iMac is on the Express one and can't connect, try connecting the PowerBook to that one and see if it has the same problem. If it does, you'll know its a problem with the config on the Express. If it doesn't, you'll know its a problem with the iMac's config, and so on...