port Forwarding

[diadicic at optonline.net]

Can someone help me understand this?

  I have a firewall setup up with two nic's in it.

               1 eth0 24.190.x.x  which is my Internet connection
               2 eth1 172.16.12.165 which is a local area network.

              I have a web server at 172.16.12.160  “I hope thats clear”

    Now if I port forward this way it will not work.

             “iptables -A PREROUTING -i eth0 -t nat -p tcp –dport 80 -j DNAT –to 
                172.16.12.160:80”
               “iptables -A FORWARD -p tcp -d 172.16.12.160 –dport 80 -i eth0 -o eth1 
                 -j ACCEPT”


But if I do it this way It will work:

                 “iptables -t nat  -A PREROUTING -p tcp –dport 80 -d 24.190.x.x -i eth0 -j   
                   DNAT --to-destination 172.16.12.160  172.16.12.160”

                   “iptables -A POSTROUTING -p tcp –dport 80 -d 172.16.12.160 -j SNAT -o    
                     eth1 –to-source 172.16.12.165”


Could anyone please explain why the first way did not work, all the directions I read claim that is how to port forward.


Any help is appreciated



Thanks 
Dominic Iadicicco