Setting up Linux Gateway

Using this, you can set up your linux pc as a home gateway for all the other systems. For instructions on how to do this, you may refer the below links:

Iptables Tutorial (look at the the Example Scripts)
Using Linux iptables or ipchains to set up an internet gateway / firewall / routerfor home or office.
Setting up a Linux Home Gateway

Please note to disable any other firewall services (such as Guarddog) if you are using your own custom rules or else the firewall will not work as expected.