To setup multiple IP addresses on the same server (and same interface) you need to input a few parameters using ipconfig. If you typer /sbin/ifconfig you will see a list of what IP addresses are asigned to what ethernet interface. You will usually see your main IP address and a loop back address. To add more IP addresses you need to use ipconfig with the following syntax:

/sbin/ifconfig eth0:0 111.222.333.444
/sbin/ifconfig eth0:1 111.222.333.445

Of course you are a dumbass if you use the IP addresses above. You will incriment the eth0:X value by 1 for each IP address. You should be able to ping your IP addresses now but after reboot your configuration is reset. The easyest way to have you configuration stay is to add the ipconfig commands to you rc.local file located at /etc/rc.d/rc.local on a redhat system.