I vaguely remember that it's possible to set up virtual network interfaces with their own IP addresses, using only one physical network connection.

I can find a few guides on the internet that recommend setting these up in /etc/network/interfaces, but Ubuntu doesn't use this file. Therefore my question:

What's the correct way of setting these up in recent versions of Ubuntu?

As this is a laptop, and I need it to connect to all kinds of different networks, I want to keep the network manager and all its configuration.

To be more clear: at the end of this, I want to have a new network interface (e.g. "eth42") with its own IP address, but using whatever is connected in network manager to send the actual packets.

In NM, it should appear as if I just had a second ethernet adapter installed in my system.