CentOS:
/etc/sysconfig/network-scripts/ifcfg-eth0
# Configuration for eth0
DEVICE=eth0
BOOTPROTO=none
# This line ensures that the interface will be brought up during boot.
ONBOOT=yes
# eth0 - This is the main IP address that will be used for most outbound connections.
# The address, netmask and gateway are all necessary.
IPADDR=12.34.56.78
NETMASK=255.255.255.0
GATEWAY=12.34.56.1