There are many times where you may need to quickly add multiple IP Addresses to a single network interface card (NIC)

Each interface is limited to 255 different ethernet devices per adaptor.

from the command line you can enter

ifconfig devicename:number ipaddress netmask yournetmask

so as root or a user with root permissions

ifconfig eth0:1 netmask

you can add this to the rc.local located in /etc/rc.d/  if you want this to take effect at boot

On Cent Os/ Redhat / Fedora

you can copy the /etc/sysconfig/network-scripts/ifcfg-eth0 to ifcfg-eth0:1

edit /etc/sysconfig/network-scripts/ifcfg-eth0:0

change the IPADDR= to your secondary address

Optionally, if you are adding disjointed ip addresses from a different network you may need to change BROADCAST, NETWORK, NETMASK, GATEWAY as well