This is a classic internet sharing problem.
My PC has 2 network cards. Eth0 to be my internet side (with fixed IP address 213.xxx/255.255.255.24 and eth1 my intranet side (192.168.0.1).
Under windows, it easy to share the internet connection (eth0) with other PCs on the intranet side.
Does any one know how to do this with knoppix?