[X4U] airport troubles

[silvo conticello]

I am just setting up my first AirPort Base Extreme.
I'm connecting to my ISP through a cable modem with a dynamic IP 
address (NTL).
The problem is that, while my AlBook is working perfectly through the 
airport card, the iMac, connected through ethernet, does not get a 
viable IP address.
The iMac gets a self-assigned Ip address that is suitable to let the 2 
computer see/connect each other, but does not work for the internet 
connection.

In the base station I've left unchecked the 'Distribute IP address' 
option (if I check it, I get in both computer the new IP address but I 
loose the connection to internet for both)
The iMac is set to use DHCP

thanks in advance,
Silvo


Silvo Conticello
Albook 17" 1.5GHz/1.5Gb RAM/80Gb HD/OS10.3.9
iMac 1GHz/500Mb Ram/80Gb HD/OS10.3.9/17" display
Cambridge UK