this has been mentioned a fair number of times on this site, please try searching in future
one way to do it is to create additional ethX files, for example you're standard eth0 config file will be
/etc/sysconfig/network-scripts/ifcfg-eth0
no you can create additional files such as ifcfg-eth0-1 etc... inside these files you just need to change the device name (and of course the IP if you are static) from eth0 to eth0:1 in this case. then the interface can be controlled with
ifup eth0-1
etc...
you can also do it instantly with ifconfig:
ifconfig eth0:0 a.b.c.d
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