An example of /etc/network/interfaces for a dual-homed host

interfaces

auto lo
iface lo inet loopback

# The 1st network of this host is 192.0.2.0/24.
# This host mainly uses the 1st network to communicate the Internet.
allow-hotplug eth0
iface eth0 inet static
    address 192.0.2.30
    netmask 255.255.255.0
    network 192.0.2.0
    broadcast 192.0.2.255
    gateway 192.0.2.1
    dns-nameservers 192.0.2.11 192.0.2.12
    dns-search example.net

# The 2nd network of this host is 198.51.100.0/24.
# Although this host does not directly connect to the 3rd network, 203.0.113.0/24,
# the router of the 2nd network knows the routing to the 3rd network.
# The IP address of the router of the 2nd network is 198.51.100.1.
allow-hotplug eth1
iface eth1 inet static
    address 198.51.100.50
    netmask 255.255.255.0
    network 198.51.100.0
    broadcast 198.51.100.255
    post-up /sbin/ip route add 203.0.113.0/24 via 198.51.100.1 src 198.51.100.50
    pre-down /sbin/ip route del 203.0.113.0/24