you could also check shorewall http://www.shorewall.net/FORWARD NAT PREROUTING POSTROUTING. im working as a trainee sir... so plz
help me to get the information regarding sir
Search for iptables tutorial or book in the net. Below link may help
you to understand the IPTables:
http://www.linuxtopia.org/Linux_Firewall_iptables/index.html