how to configure NAT with iptables

this guide will show step by step how to configure nat on your network

http://www.howtoforge.com/nat_iptables

or this http://www.cahilig.net/2008/06/09/make-your-centos-4-box-acting-internet-gatewayrouter-using-ip-masquerading

http://lists.centos.org/pipermail/centos-virt/2009-April/000964.html

http://blog.up-link.ro/freebsd-how-to-create-a-network-bridge/

http://www.rootninja.com/create-a-network-bridge-for-virtual-machines/