Simple it is. Assign an IP address and bring the interface up.
Code:
ifconfig eth0 ${IP_ADDR} broadcast ${BROADCAST} netmask ${NETMASK} up
Set route.
Code:
route add default gw ${GATEWAY}
Done! To access internet you need to configure DNS, too. Put a nameserver into /etc/resolv.conf and you are good to go. Of course, to make this setup stick and survive reboot you need to edit configuration file(s). Which ones I do not know because you chose not to reveal what Linux distro you are using.
