osx man page for lo

LO(4)							   BSD Kernel Interfaces Manual 						     LO(4)

NAME
     lo -- software loopback network interface

SYNOPSIS
     pseudo-device
     loop

DESCRIPTION
     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 hard-
     ware interfaces exist.

DIAGNOSTICS
     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.

SEE ALSO
     inet(4), intro(4), ns(4)

HISTORY
     The lo device appeared in 4.2BSD.

BUGS
     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.

4.2 Berkeley Distribution					   June 5, 1993 					 4.2 Berkeley Distribution