2.9BSD/usr/net/man/man4/lo.4

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.TH LO 4 "26 March 1982"
.UC 4
.SH NAME
lo \- software loopback network interface
.SH SYNOPSIS
.B pseudo-device loop
.SH DESCRIPTION
The
.I loop
interface is a software loopback mechanism which may be
used for performance analysis, software testing, and/or local
communication.   The loopback interface is accessible as
network 127 (non-standard).
.SH DIAGNOSTICS
\fBlo%d: can't handle af%d\fP.  The interface was handed
a message with addresses formatted in an unsuitable address
family; the packet was dropped.
.SH SEE ALSO
intro(4N), inet(4F)
.SH BUGS
It should handle all address and protocol families.
An approved network address should be reserved for this
interface.