edit /etc/network/interface.

and example

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 172.20.10.101
        netmask 255.255.255.0
        network 172.20.10.0
        broadcast 172.20.10.255
        gateway 172.20.10.100

auto eth1
iface eth1 inet static
        address 192.168.1.1
        netmask 255.255.255.0
        network 192.168.1.0
        broadcast 192.168.1.255

or man interfaces .