I am a beginner in linux platform. I have installed the ubuntu 12.04 lts. Now I want internet connection sharing between the two lan cards. So please help me.
Internet connection sharing in 12.04 is plug-n-play. However, there may be exceptions. Could you clarify your setup some more, like - What is the Internet connection? Which machine is connected to Internet and how? What are the connectivity options on the peer computers that need to connect via the share?
Basically, on the computer that will be connected to the Internet, you have to edit the LAN connection and select the IPv4 Settings method - "Shared to other computers" and also check "Available to all users". See below image.
Now, as soon as you hookup the peer computer to the above LAN connection, Internet is available.