It seems that u need a router or gateway to connect ur two lans.
I don't know how to setup a UNIX box as a gateway, but i ever did it in a windows.
In ur case, Lan1 is connected to internet through router and set NAT in the router or fireware.
Lan2 is another lan which u want to connect to internet.
U can setup a windows with two nics, lets call it Gateway. First nic is nicA, the second one is nicB.
Connect nicA in lan1, assign it the ip address and default gateway of lan1.
Connect nicB in lan2, assign it the ip address of lan2, leave the default gateway blank.
Install Winroute in ur Gateway, inside Winroute, enable NAT in nicA.
Set all other computers in lan2 to use the ip address of nicB as their default gateway.
Then all computers in lan2 can connect internet through the Gateway.


I am very interesting in how to setup a UNIX box as a gateway and enable NAT, any one can give me an example? Thanks