You set static routes in /etc/network/interfaces An example entry would be

auto eth0

allow-hotplug eth0

iface eth0 inet static

        address 10.70.201.5

        netmask 255.255.255.192

        ### Ubuntu Linux add persistent route command ###

        post-up route add -net 10.0.0.0 netmask 255.0.0.0 gw 10.70.201.6