Multihomed nics.

More and more people are wanting to separate their private network from the internet but still want to access both for a system or two. No problem. You can set up multihomed nics faily easily and all things being equal, no packets should go between the two.

If you have 2 NIC (network Lan card) each connected to different networks:

=> eth0: 192.168.1.0/24
=> eth1: 192.168.2.0/24

Consider above setup. Now if you want to route traffic to connected network only (eth0 and eth1) w/o setting Linux server as a router. This is generally called multi homing setup. You don't have to setup Linux box as a router to use multi homing :). Just assign as IP address to each NIC and you are done.

How do I configure Linux multi homing?

Just assign each interface IP address using ifconfig or use DHCP and multi homing will be turned on:

# ifconfig eth0 192.168.1.254 up
# ifconfig eth1 192.168.2.254 up

Commonly, most admin confuse the idea of multi-homing with the concept of routing or IP forwarding. Once two IP address assigned your system follows the default. No special configuration needed. Your multi-homing is up and running :).

 Private network setup:
http://www.instructables.com/id/Your-personal-intranet-Part-1/
http://www.instructables.com/id/Your-personal-intranet-Part-2/