What you are wanting to setup is a sub-interface, you want to edit your /etc/network/interfaces
. Lets say your interface setup looks something like this.
iface eth0 inet static
iface eth0:0 inet static
The subinterface would or could look like this. Note: this is for a static subinterface, you could make it DHCP if you wanted. Notice the first line, there is no rule that says you have to set it up as eth0:0 you could have it eth0:1 or 2, but I think it is easier to start the sub with a 0.
Also the subs IP address can not be already in use.