Bind Multiple IPs To One Physical Interface on Mac OS X

Via the System Preferences: Network pane

  • Go to System PreferencesNetwork.
  • Click on Ethernet (or AirPort)
  • Click on the Configuration WheelDuplicate Service...

You can now configure each instance with different settings, and you're done.

On the Terminal

If you just need to quickly assign an IP/subnet alias to an interface, you can do it with the ifconfig command in the Terminal.

sudo ifconfig <if> alias

where is the interface you want to add an address to (en0, en1, etc).