Networking :: Share Internet Connection

Is there any way I can share an internet connection with DSL?
I have three ethernet cards installed. Eth0 connects to the modem, but I would like eth1 and eth2 to allow other computers to connect to the modem.

Sure. Just download iptables.dsl and read the iptables-HOWTO at tldp.org.
The section for net sharing is called IP masquerading.

original here.