lo —
software loopback network interface
The loop interface is a software loopback mechanism
  which may be used for performance analysis, software testing, and/or local
  communication. As with other network interfaces, the loopback interface must
  have network addresses assigned for each address family with which it is to be
  used. These addresses may be set or changed with the
  SIOCSIFADDR
  ioctl(2). The loopback interface
  should be the last interface configured, as protocols may use the order of
  configuration as an indication of priority. The loopback should
  never be configured first unless no hardware interfaces
  exist.
The loopback interface lo0 is created at
    boottime, it always exists and cannot be destroyed with
    ifconfig(8). Additional
    loopback interfaces can be created by using the
    ifconfig(8)
    create command.
  - lo%d: can't handle af%d.
- The interface was handed a message with addresses formatted in an
      unsuitable address family; the packet was dropped.
Thelo device appeared in
  4.2BSD.
Previous versions of the system enabled the loopback interface automatically,
  using a nonstandard Internet address (127.1). Use of that address is now
  discouraged; a reserved host address for the local network should be used
  instead.