I know that you can make a virtual network interface in Windows (see here), and in Linux it is also pretty easy with ip-aliases, but does something similar exist for Mac OS X? I've been looking for loopback adapters, virtual interfaces and couldn't find a good solution.
You can create a new interface in the networking panel, based on an existing interface, but it will not act as a real fully functional interface (if the original interface is inactive, then the derived one is also inactive).
This scenario is needed when working in a completely disconnected situation. Even then, it makes sense to have networking capabilities when running servers in a VMWare installation. Those virtual machines can be reached by their IP address, but not by their DNS name, even if I run a DNS server in one of those virtual machines. By configuring an interface to use the virtual DNS server, I thought I could test some DNS scenario's. Unfortunately, no interface is resolving DNS names if none of them are inactive…
Best Answer
The loopback adapter is always up.
ifconfig lo0 alias 172.16.123.1 will add an alias IP 172.16.123.1 to the loopback adapter
ifconfig lo0 -alias 172.16.123.1 will remove it