This will be part 1 in a series of configuring CentOS/Red Hat 6 as a secured firewall. Though I am a huge fan of pfSense (which can be found here pfSense), I wanted to build my own from scratch. So, the first part of this series will consist of setting up PAT (or NAT overload for the Cisco geeks) on Linux.
The first step is to configure the network cards. In this scenario we will use eth0 as the WAN connection and eth1 as the LAN connection. Refer to the diagram below