Check out ifup-aliases in /etc/sysconfig/network-scripts if you're running RedHat. Follow the instructions and define an ifcfg-eth0-range0 file with the block of aliases you want defined for eth0 in addition to the address defined in ifcfg-eth0.
Then ifconfig eth0 down, ifconfig eth0 up and you should be set. The ifup-aliases script does all the work of setting up the routes etc that are needed to make multiple IPs on a single NIC work. If you do an ifconfig eth0 you'll see eth0:1 eth0:2 etc in addition to eth0, with the proper IPs, if ifconfig up worked properly.
