If you run Debian or Ubuntu
Edit /etc/network/interfaces
auto eth0:0 iface eth0:0 inet static address X.X.X.X netmask 255.255.255.0
For Centos
edit
/etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE=eth0:0 BOOTPROTO=none ONPARENT=yes IPADDR=XXX.XXX.XXX.XXX NETMASK=255.255.255.0