I have set 3 ubuntu12.0.4 virtual machines installed on vmplayer. Out of which I have made two of them as host of two different network. Host1 ip=192.168.1.13 gateway=192.168.1.11 and Host2 ip=192.168.2.14 and gateway=192.168.2.12 . Now I want the third virtual machine as router which can transfer packets from first virtual machine to second virtual machine has two adapters eth0 ip=192.168.1.11 and eth1 ip=192.168.2.12 . Host1 is connected to eth0 and Host2 is connected to eth1. How shall I configure the router so that the two hosts can communicate and NAT'ing could be performed?

There are a number of ways. In your question I don't think the vm part is relevant. (could be though)

See this post for two ways that may work. http://serverfault.com/questions/611...-network-cards

It may or may not be what you are asking about.

Not sure there is a great reason to do what you want other than to test real setup or learn. In a normal vm setup there are other ways to get this to work.