Description:

netplug is a daemon that manages network interfaces in response to
link-level events such as cables being plugged in and out. When a
cable is plugged into an interface, the netplug daemon brings that
interface up. When the cable is unplugged, the daemon brings that
interface back down.

This is extremely useful for systems such as laptops, which are
constantly being unplugged from one network and plugged into another,
and for moving systems in a machine room from one switch to another
without a need for manual intervention.