iMac connects via Airport but not ethernet?

I want to use our stalwart iMac G4 as an FTP server running Rumpus. I can connect to our Windows domain and browse the web when connected via Airport, but can't when connecting via ethernet. I can ping our domain controller while using ethernet and it can ping back. I changed the IP address for the ethernet connection, but still no luck.
I can't tell if there's maybe a problem with the iMac's NIC or if it's a configuration issue with the domain server.
Any ideas?

thanks!