Login as root then do the following to add a new ip address to any linux redhat based system. If you have cpanel there is a nice little ip management script that just lets you do it via WHM. For those without cpanel do the following: 

cd /etc/sysconfig/network-scripts

Then as a usual precaution I would make a backup of the file
cp ifcfg-eth0 ifcfg-eth0.bak
(you wont need to do anything with the backup)

Then copy the config for your new IP
cp ifcfg-eth0 ifcfg-eth0:1

Then we need to edit the new config
pico -w ifcfg-eth0:1

The lines you need to change:

DEVICE="eth0"
to
DEVICE="eth0:1"

Then change the
IPADDR="xxx.xxx.xxx.xxx"
to be your NEW ip address (it will have your primary ip listed)

Once this is done, press CTRL + X
then press Y and

This will have saved the config for you and the interface is ready to be brought up. This is done with
/sbin/ifup eth0:1

Having done this if you type
/sbin/ifconfig

You should see something like
------code------
eth0 Link encap:Ethernet HWaddr 00:00:00:00:00:00
inet addr:xxx.xxx.xxx.xxx Bcast:xxx.xxx.xxx.xxx Mask:255.255.254.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:55818546 errors:0 dropped:0 overruns:0 frame:0
TX packets:46167836 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:3299680715 (3146.8 Mb) TX bytes:1890963825 (1803.3 Mb)
Interrupt:11 Base address:0xd000

eth0:1 Link encap:Ethernet HWaddr 00:00:00:00:00:00
inet addr:NEW.IP.ADDRESS Bcast:xxx.xxx.xxx.xxx Mask:255.255.254.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:11 Base address:0xd000

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:241244 errors:0 dropped:0 overruns:0 frame:0
TX packets:241244 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:66571100 (63.4 Mb) TX bytes:66571100 (63.4 Mb)
------/code------

If you then try to ping your new ip it should all work fine 
This should also restart eth0:1 on a reboot as its a direct copy of the eth0 config. So checking for the ONBOOT="yes" in ifcfg-eth0:1 will ensure it DOES come back up on a reboot.

This guide was originally posted at: http://forums.rackshack.net/showthread.php?s=4834c29624a4755c87defe9fa144c29a&threadid=19915&highlight=eth01