Loopback Interface

All systems have a “loopback” interface, regardless of whether they are connected to the network or not. This is not a true network interface, but acts as one to allow some network-aware programs (such as RPC) to be able to connect to both the remote and local host in the same way.

We don't need to be concerned with configuring it, as this is performed by the operating system, but we do need to make certain that the local host address remains in the host file. We can display the current loopback settings using ifconfig:

hydrogen# ifconfig lo0
lo0: flags=849<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232
        inet 127.0.0.1 netmask ff000000
hydrogen#

We can see that this has a netmask of 255.0.0.0 (FF000000 in hex), as we would expect from a class “A” address. The address should be defined as 127.0.0.1 on every host.