sudo cat /etc/network/interfaces
The output should show something like this
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
auto eth0 eth1
iface eth0 inet static
address 192.168.1.2
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.254
# dns-* options are implemented by the resolvconf package, if installed
iface eth1 inet static
address 172.20.10.1
netmask 255.255.255.0
broadcast 172.20.10.255
gateway 172.20.10.254
# static route
up route add -net 172.20.11.0/16 gw 172.20.10.254 dev eth1
http://www.ubuntugeek.com/howto-add-permanent-static-routes-in-ubuntu.html