HOW TO ADD ADDITIONAL IP TO A NETWORK INTERFACE ON MAC OS X

Source: https://gerrydevstory.com/2012/08/20/how-to-create-virtual-network-interface-on-mac-os-x/

To add an additional IP to a network interface on OSX using the GUI simply “replicate service” on any interface via the cog in [Network Settings] and then assign the IP to the replicated interface manually.

source: http://hints.macworld.com/article.php?story=20041223112819235

From the terminal:

sudo ifconfig en1 inet xxx.xxx.xxx.xxx netmask 255.255.255.0 alias

Where sudo = execute as root, en1 = the interface you want to add an alias to, xxx.xxx.xxx.xxx = ip you wan to add, 255.255.255.0 = desired netmask.