<s>
Loopback	B-Protocol
(	O
also	O
written	O
loop-back	B-Protocol
)	O
is	O
the	O
routing	O
of	O
electronic	O
signals	O
or	O
digital	O
data	O
streams	O
back	O
to	O
their	O
source	O
without	O
intentional	O
processing	O
or	O
modification	O
.	O
</s>
<s>
It	O
may	O
be	O
a	O
communication	O
channel	O
with	O
only	O
one	O
communication	B-Protocol
endpoint	I-Protocol
.	O
</s>
<s>
In	O
telecommunications	O
,	O
loopback	B-Protocol
devices	O
perform	O
transmission	O
tests	O
of	O
access	O
lines	O
from	O
the	O
serving	O
switching	O
center	O
,	O
which	O
usually	O
does	O
not	O
require	O
the	O
assistance	O
of	O
personnel	O
at	O
the	O
served	O
terminal	O
.	O
</s>
<s>
A	O
patch	B-Device
cable	I-Device
may	O
also	O
function	O
as	O
loopback	B-Protocol
,	O
when	O
applied	O
manually	O
or	O
automatically	O
,	O
remotely	O
or	O
locally	O
,	O
facilitating	O
a	O
loop-back	B-Protocol
test	O
.	O
</s>
<s>
Where	O
a	O
system	O
(	O
such	O
as	O
a	O
modem	B-Application
)	O
involves	O
round-trip	O
analog-to-digital	O
processing	O
,	O
a	O
distinction	O
is	O
made	O
between	O
analog	B-Protocol
loopback	I-Protocol
,	O
where	O
the	O
analog	O
signal	O
is	O
looped	O
back	O
directly	O
,	O
and	O
digital	B-Protocol
loopback	I-Protocol
,	O
where	O
the	O
signal	O
is	O
processed	O
in	O
the	O
digital	O
domain	O
before	O
being	O
re-converted	O
to	O
an	O
analog	O
signal	O
and	O
returned	O
to	O
the	O
source	O
.	O
</s>
<s>
In	O
telecommunications	O
,	O
loopback	B-Protocol
,	O
or	O
a	O
loop	O
,	O
is	O
a	O
hardware	O
or	O
software	O
method	O
which	O
feeds	O
a	O
received	O
signal	O
or	O
data	O
back	O
to	O
the	O
sender	O
.	O
</s>
<s>
This	O
is	O
called	O
a	O
loopback	B-Protocol
test	O
and	O
can	O
be	O
performed	O
within	O
a	O
modem	B-Application
or	O
transceiver	O
by	O
connecting	O
its	O
output	O
to	O
its	O
own	O
input	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
network	O
termination	O
connector	O
such	O
as	O
X.21	B-Protocol
,	O
this	O
is	O
typically	O
done	O
by	O
simply	O
connecting	O
the	O
pins	O
together	O
in	O
the	O
connector	O
.	O
</s>
<s>
Media	O
such	O
as	O
optical	B-Architecture
fiber	I-Architecture
or	O
coaxial	O
cable	O
,	O
which	O
have	O
separate	O
transmit	O
and	O
receive	O
connectors	O
,	O
can	O
simply	O
be	O
looped	O
together	O
with	O
a	O
single	O
strand	O
of	O
the	O
appropriate	O
medium	O
.	O
</s>
<s>
A	O
modem	B-Application
can	O
be	O
configured	O
to	O
loop	O
incoming	O
signals	O
from	O
either	O
the	O
remote	O
modem	B-Application
or	O
the	O
local	O
terminal	O
.	O
</s>
<s>
This	O
is	O
referred	O
to	O
as	O
loopback	B-Protocol
or	O
software	O
loop	O
.	O
</s>
<s>
A	O
serial	O
communications	O
transceiver	O
can	O
use	O
loopback	B-Protocol
for	O
testing	O
its	O
functionality	O
.	O
</s>
<s>
Moving	O
it	O
to	O
the	O
far	O
end	O
of	O
a	O
modem	B-Application
link	O
extends	O
the	O
test	O
further	O
.	O
</s>
<s>
Some	O
devices	O
include	O
built-in	O
loopback	B-Protocol
capability	O
.	O
</s>
<s>
A	O
simple	O
serial	O
interface	O
loopback	B-Protocol
test	O
,	O
called	O
paperclip	O
test	O
,	O
is	O
sometimes	O
used	O
to	O
identify	O
serial	O
ports	O
of	O
a	O
computer	O
and	O
verify	O
operation	O
.	O
</s>
<s>
For	O
this	O
purpose	O
,	O
a	O
paperclip	O
is	O
used	O
to	O
short	O
pin	O
2	O
to	O
pin	O
3	O
(	O
the	O
receive	O
and	O
transmit	O
pins	O
)	O
on	O
a	O
standard	O
RS-232	O
interface	O
using	O
D-subminiature	B-Protocol
DE-9	O
or	O
DB-25	B-Protocol
connectors	I-Protocol
.	O
</s>
<s>
Implementations	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
include	O
a	O
virtual	B-General_Concept
network	I-General_Concept
interface	I-General_Concept
through	O
which	O
network	O
applications	O
can	O
communicate	O
when	O
executing	O
on	O
the	O
same	O
machine	O
.	O
</s>
<s>
It	O
is	O
implemented	O
entirely	O
within	O
the	O
operating	O
system	O
's	O
networking	O
software	O
and	O
passes	O
no	O
packets	O
to	O
any	O
network	B-Protocol
interface	I-Protocol
controller	I-Protocol
.	O
</s>
<s>
Any	O
traffic	O
that	O
a	O
computer	O
program	O
sends	O
to	O
a	O
loopback	B-Protocol
IP	I-Protocol
address	I-Protocol
is	O
simply	O
and	O
immediately	O
passed	O
back	O
up	O
the	O
network	O
software	O
stack	O
as	O
if	O
it	O
had	O
been	O
received	O
from	O
another	O
device	O
.	O
</s>
<s>
Unix-like	B-Operating_System
systems	I-Operating_System
usually	O
name	O
this	O
loopback	B-Protocol
interface	O
lo	O
or	O
lo0	B-Protocol
.	O
</s>
<s>
Various	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
standards	O
reserve	O
the	O
IPv4	O
address	O
block	O
127.0.0.0/8	B-Protocol
,	O
in	O
CIDR	B-Protocol
notation	I-Protocol
and	O
the	O
IPv6	O
address	O
::1/128	O
for	O
this	O
purpose	O
.	O
</s>
<s>
The	O
most	O
common	O
IPv4	O
address	O
used	O
is	O
127.0.0.1	B-Protocol
.	O
</s>
<s>
Commonly	O
these	O
loopback	B-Protocol
addresses	O
are	O
mapped	O
to	O
the	O
hostnames	O
localhost	B-Protocol
or	O
loopback	B-Protocol
.	O
</s>
<s>
One	O
notable	O
exception	O
to	O
the	O
use	O
of	O
the	O
127.0.0.0/8	B-Protocol
network	O
addresses	O
is	O
their	O
use	O
in	O
Multiprotocol	B-Protocol
Label	I-Protocol
Switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
traceroute	O
error	O
detection	O
,	O
in	O
which	O
their	O
property	O
of	O
not	O
being	O
routable	O
provides	O
a	O
convenient	O
means	O
to	O
avoid	O
delivery	O
of	O
faulty	O
packets	O
to	O
end	O
users	O
.	O
</s>
<s>
Any	O
IP	O
datagram	B-Protocol
with	O
a	O
source	O
or	O
destination	O
address	O
set	O
to	O
a	O
loopback	B-Protocol
address	I-Protocol
must	O
not	O
appear	O
outside	O
of	O
a	O
computing	O
system	O
,	O
or	O
be	O
routed	O
by	O
any	O
routing	O
device	O
.	O
</s>
<s>
Packets	O
received	O
on	O
an	O
interface	O
with	O
a	O
loopback	B-Protocol
destination	O
address	O
must	O
be	O
dropped	O
.	O
</s>
<s>
Such	O
packets	O
are	O
sometimes	O
referred	O
to	O
as	O
Martian	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
As	O
with	O
other	O
bogus	O
packets	O
,	O
they	O
may	O
be	O
malicious	O
and	O
any	O
problems	O
they	O
might	O
cause	O
can	O
be	O
avoided	O
by	O
applying	O
bogon	B-Protocol
filtering	I-Protocol
.	O
</s>
<s>
Some	O
computer	O
network	O
equipment	O
use	O
the	O
term	O
"	O
loopback	B-Protocol
"	O
for	O
a	O
virtual	B-General_Concept
interface	I-General_Concept
used	O
for	O
management	O
purposes	O
.	O
</s>
<s>
Unlike	O
a	O
proper	O
loopback	B-Protocol
interface	O
,	O
this	O
type	O
of	O
loopback	B-Protocol
device	O
is	O
not	O
used	O
to	O
talk	O
with	O
itself	O
.	O
</s>
<s>
Such	O
a	O
loopback	B-Protocol
device	O
is	O
also	O
used	O
for	O
management	O
datagrams	B-Protocol
,	O
such	O
as	O
alarms	O
,	O
originating	O
from	O
the	O
equipment	O
.	O
</s>
<s>
The	O
property	O
that	O
makes	O
this	O
virtual	B-General_Concept
interface	I-General_Concept
special	O
is	O
that	O
applications	O
that	O
use	O
it	O
will	O
send	O
or	O
receive	O
traffic	O
using	O
the	O
address	O
assigned	O
to	O
the	O
virtual	B-General_Concept
interface	I-General_Concept
as	O
opposed	O
to	O
the	O
address	O
on	O
the	O
physical	O
interface	O
through	O
which	O
the	O
traffic	O
passes	O
.	O
</s>
<s>
Loopback	B-Protocol
interfaces	O
of	O
this	O
sort	O
are	O
often	O
used	O
in	O
the	O
operation	O
of	O
routing	B-Protocol
protocols	I-Protocol
,	O
because	O
they	O
have	O
the	O
useful	O
property	O
that	O
,	O
unlike	O
real	O
physical	O
interfaces	O
,	O
they	O
will	O
not	O
go	O
down	O
when	O
a	O
physical	O
port	O
fails	O
.	O
</s>
<s>
The	O
audio	O
systems	O
Open	B-Library
Sound	I-Library
System	I-Library
(	O
OSS	O
)	O
,	O
Advanced	B-Application
Linux	I-Application
Sound	I-Application
Architecture	I-Application
(	O
ALSA	B-Application
)	O
and	O
PulseAudio	B-Library
have	O
loopback	B-Protocol
modules	O
for	O
recording	O
the	O
audio	O
output	O
of	O
applications	O
for	O
testing	O
purposes	O
.	O
</s>
<s>
Unlike	O
physical	O
loopbacks	B-Protocol
,	O
this	O
does	O
not	O
involve	O
double	O
analog/digital	O
conversion	O
and	O
no	O
disruption	O
is	O
caused	O
by	O
hardware	O
malfunctions	O
.	O
</s>
