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 eth0:1 192.168.1.10 netmask 255.255.255.0
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