Sub interfaces or virtual interfaces are used for a number of reasons. Normally for VLANs, but also if you want your machine to have multiple IP addresses.
This is relatively straight forward to do.
It can be done from the command line like this:
# ifconfig eth0:1 192.168.111.1
The above command has just created a virtual / sub interface on eth0 called eth0:1 and assigned it the IP 192.168.111.1
This however is not a permanent solution because when you reboot, this interface will be lost. To make it permanent we need to create a file in /etc/sysconfig/network-scripts/ called ifcfg-eth0:1
DEVICE=eth0:1 BOOTPROTO=none HWADDR=00:16:17:90:a5:15 ONBOOT=yes IPADDR=192.168.111.1 NETMASK=255.255.255.0 TYPE=Ethernet
Very similar to ifcfg-eth0 but note there is no default gateway set. Always remove the gateway line from the cfg file you will inevitably copy to create this.
The MAC or Hardware address must also match the parent interface.
If you need more than one virtual / sub interface, simply create more config files.
To bring an interface up after creating the config file use:
# ifup eth0:1
