I want to create routing for my two Lan Card. First Lan Card (eth0) for Internet access, and second Lan Card (eth1) for my Intranet. So when i access internet, it will route to my eth0 by default. otherwise, with eth1 for intranet access.

Public (eth0):
IP: 201.102.21.201
Netmask: 255.255.255.241
Gateway: 201.102.21.1

Intranet (eth1):
IP: 10.1.10.24
Netmask: 255.255.255.0
Gateway: 10.1.10.1

Basic command for adding route on Linux:

route add [-net|-host] <IP/Net> netmask <Mask> gw <Gateway IP> dev <Int>X

And this is what we do:

route add -net 10.0.0.0 netmask 255.0.0.0 gw 10.1.10.1 dev eth1
route add -net default netmask 0.0.0.0 gw 201.102.21.1 dev eth0

To check the route, simply type “route” with no arguments

route

The output will seen like this:

Kernel IP routing table
Destination¬ ¬ ¬ ¬ Gateway¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Genmask¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Flags¬ Metric Ref¬ ¬ ¬ Use Iface
201.102.21.201¬ *¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 255.255.255.241¬ ¬ ¬ ¬ U¬ ¬ ¬ ¬ 1¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ 0 eth0
10.1.10.24¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ *¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 255.255.255.0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ U¬ ¬ ¬ ¬ ¬ 1¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ 0 eth1
link-local¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ *¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 255.255.0.0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ U¬ ¬ ¬ 1000¬ ¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ 0 eth0
10.0.0.0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 10.1.10.1¬ ¬ ¬ ¬ ¬ ¬ 255.0.0.0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ UG¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ 0 eth1
default¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 201.102.21.1¬ ¬ ¬ 0.0.0.0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ UG¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 0¬ ¬ ¬ ¬ ¬ ¬ ¬ 0 eth0

Now, to keep this route persistent, add this lines to file /etc/networking/interfaces

up route add -net 10.0.0.0 netmask 255.0.0.0 gw 10.1.10.1 dev eth1
up route add -net default netmask 0.0.0.0 gw 201.102.21.1 dev eth0

Restart networking to confirm the changes

sudo /etc/init.d/networking restart

Other method, Create a file called defaultroute in /etc/init.d and make it executable (chmod +x /etc/init.d/defaultroute).
Add the route commands there and do this:

update-rc.d defaultroute defaults 99

.