- Hey guys what is IP Forwarding and how do you set it up in Centos and UBUNTU?

IP Forwarding allows traffic from one network to be passed to another network. In other words, a machine with IP forwarding enabled can act as a router.

For example traffic from 192.168.1.0/24 will be allowed to pass to 192.168.2.0/24 (all traffic is still subject to iptables rules ofcourse).

To turn IP forwarding on in Ubuntu use:
#echo 1 > /proc/sys/net/ipv4/ip_forward

and to turn it off use:
#echo 0 > /proc/sys/net/ipv4/ip_forward

Note that there is a config file you can edit to enable/disable ip forwarding whenever the machine starts, but i can't remember the path right now

Finally, you'll need to add the proper routes to the kernel routing table so the machine nows what traffic to forward and where to forward it to.
For this you can #man route

Cheers

What if I were to forward packets within the same LAN? Do I still need to enable IP forwarding?

Do you mean /etc/sysctl.conf? If you put the following in that file and run sysctl -p it has the same effect:
If your box only has one NIC in it, it sounds like iptables would be a better choice.

- is it possible to use IP Forwading to connect different networks and do you have to learn iptables if you are dealing with IP Forwarding.

If you have IP forwarding on and IP tables off, two nics on the machine, one connected to one network and other connected to the second network it will work.

However, if you are not using ip tables you will get traffic (read access and insecure)both ways.

k and other connected to the second network it will work.
- so how do i configure IP Forwarding to connect 2 different networks.

just search this site of google for hundreds of identical questions. indeed just look at the bottom of this page in the similar threads box.