<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
link-local	B-Protocol
address	I-Protocol
is	O
a	O
unicast	B-Protocol
network	I-Protocol
address	I-Protocol
that	O
is	O
valid	O
only	O
for	O
communications	O
within	O
the	O
subnetwork	B-Protocol
that	O
the	O
host	O
is	O
connected	O
to	O
.	O
</s>
<s>
Link-local	O
addresses	O
are	O
most	O
often	O
assigned	O
automatically	O
with	O
a	O
process	O
known	O
as	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
or	O
link-local	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
,	O
also	O
known	O
as	O
automatic	O
private	O
IP	O
addressing	O
(	O
APIPA	B-Protocol
)	O
or	O
auto-IP	B-Protocol
.	O
</s>
<s>
Therefore	O
,	O
routers	B-Protocol
do	O
not	O
forward	O
packets	B-Protocol
with	O
link-local	O
source	O
or	O
destination	O
addresses	O
.	O
</s>
<s>
IPv4	B-Protocol
link-local	O
addresses	O
are	O
assigned	O
from	O
address	O
block	O
(	O
through	O
)	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
they	O
are	O
assigned	O
from	O
the	O
block	O
.	O
</s>
<s>
In	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
networks	O
,	O
they	O
are	O
assigned	O
most	O
often	O
using	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
,	O
a	O
process	O
that	O
often	O
uses	O
a	O
stochastic	O
process	O
to	O
select	O
the	O
value	O
of	O
link-local	O
addresses	O
,	O
assigning	O
a	O
pseudo-random	O
address	O
that	O
is	O
different	O
for	O
each	O
session	O
.	O
</s>
<s>
However	O
,	O
in	O
IPv6	B-Protocol
the	O
link-local	B-Protocol
address	I-Protocol
may	O
be	O
derived	O
from	O
the	O
interface	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
MAC	O
)	O
address	O
in	O
a	O
rule-based	O
method	O
.	O
</s>
<s>
In	O
IPv4	B-Protocol
,	O
link-local	O
addresses	O
are	O
normally	O
only	O
used	O
when	O
no	O
external	O
,	O
stateful	O
mechanism	O
of	O
address	O
configuration	O
exists	O
,	O
such	O
as	O
the	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
(	O
DHCP	B-Protocol
)	O
,	O
or	O
when	O
another	O
primary	O
configuration	O
method	O
has	O
failed	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
link-local	O
addresses	O
are	O
always	O
assigned	O
,	O
along	O
with	O
addresses	O
of	O
other	O
scopes	O
,	O
and	O
are	O
required	O
for	O
the	O
internal	O
functioning	O
of	O
various	O
protocol	O
components	O
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
has	O
reserved	O
the	O
IPv4	B-Protocol
address	O
block	O
(	O
)	O
for	O
link-local	B-Protocol
addressing	I-Protocol
.	O
</s>
<s>
stateless	O
,	O
address	B-Protocol
autoconfiguration	I-Protocol
when	O
other	O
means	O
of	O
address	O
assignment	O
are	O
not	O
available	O
.	O
</s>
<s>
The	O
simultaneous	O
use	O
of	O
IPv4	B-Protocol
addresses	O
of	O
different	O
scope	O
on	O
the	O
same	O
interface	O
,	O
such	O
as	O
configuring	O
link-local	O
addresses	O
as	O
well	O
as	O
globally	O
routable	O
addresses	O
,	O
may	O
lead	O
to	O
confusion	O
and	O
increased	O
complexity	O
.	O
</s>
<s>
Therefore	O
,	O
hosts	O
search	O
for	O
a	O
DHCP	B-Protocol
server	I-Protocol
on	O
the	O
network	O
before	O
assigning	O
link-local	O
addresses	O
.	O
</s>
<s>
When	O
a	O
globally	O
routable	O
or	O
a	O
private	B-Protocol
address	I-Protocol
becomes	O
available	O
after	O
a	O
link-local	B-Protocol
address	I-Protocol
has	O
been	O
assigned	O
,	O
the	O
use	O
of	O
the	O
new	O
address	O
should	O
generally	O
be	O
preferred	O
to	O
the	O
link-local	B-Protocol
address	I-Protocol
for	O
new	O
connections	O
but	O
communication	O
via	O
the	O
link-local	B-Protocol
address	I-Protocol
is	O
still	O
possible	O
.	O
</s>
<s>
Microsoft	O
refers	O
to	O
this	O
address	B-Protocol
autoconfiguration	I-Protocol
method	O
as	O
Automatic	O
Private	O
IP	O
Addressing	O
(	O
APIPA	B-Protocol
)	O
.	O
</s>
<s>
In	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
,	O
the	O
address	O
block	O
has	O
been	O
reserved	O
for	O
link-local	O
unicast	B-Protocol
addressing	O
.	O
</s>
<s>
Of	O
the	O
64	O
bits	O
of	O
a	O
link-local	O
addresses	O
 '	O
network	O
component	O
,	O
the	O
most	O
significant	O
10	O
bits	O
(	O
1111111010	O
)	O
correspond	O
to	O
the	O
IANA-reserved	O
"	O
global	O
routing	O
prefix	O
"	O
for	O
link-local	O
addresses	O
,	O
while	O
the	O
"	O
subnet	B-Protocol
ID	O
"	O
(	O
the	O
remaining	O
54	O
bits	O
)	O
is	O
zero	O
.	O
</s>
<s>
Unlike	O
IPv4	B-Protocol
,	O
IPv6	B-Protocol
requires	O
a	O
link-local	B-Protocol
address	I-Protocol
on	O
every	O
network	O
interface	O
on	O
which	O
the	O
IPv6	B-Protocol
protocol	O
is	O
enabled	O
,	O
even	O
when	O
routable	O
addresses	O
are	O
also	O
assigned	O
.	O
</s>
<s>
Consequently	O
,	O
IPv6	B-Protocol
hosts	O
usually	O
have	O
more	O
than	O
one	O
IPv6	B-Protocol
address	I-Protocol
assigned	O
to	O
each	O
of	O
their	O
IPv6-enabled	O
network	O
interfaces	O
.	O
</s>
<s>
The	O
link-local	B-Protocol
address	I-Protocol
is	O
required	O
for	O
IPv6	B-Protocol
sublayer	O
operations	O
of	O
the	O
Neighbor	O
Discovery	O
Protocol	O
,	O
as	O
well	O
as	O
for	O
some	O
other	O
IPv6-based	O
protocols	O
,	O
such	O
as	O
DHCPv6	B-Protocol
.	O
</s>
<s>
When	O
using	O
an	O
IPv6	B-Protocol
link-local	B-Protocol
address	I-Protocol
to	O
connect	O
to	O
a	O
host	O
,	O
a	O
zone	O
index	O
must	O
be	O
added	O
to	O
the	O
address	O
so	O
that	O
the	O
packets	B-Protocol
can	O
be	O
sent	O
out	O
on	O
the	O
correct	O
interface	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
addresses	O
may	O
be	O
assigned	O
by	O
stateless	O
(	O
automatic	O
)	O
or	O
stateful	O
(	O
manual	O
)	O
mechanisms	O
.	O
</s>
<s>
Stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
is	O
performed	O
as	O
a	O
component	O
of	O
the	O
Neighbor	O
Discovery	O
Protocol	O
(	O
NDP	O
)	O
.	O
</s>
<s>
Through	O
NDP	O
routing	O
prefix	O
advertisements	O
,	O
a	O
router	B-Protocol
or	O
server	O
host	O
may	O
announce	O
configuration	O
information	O
to	O
all	O
link-attached	O
interfaces	O
which	O
causes	O
additional	O
IP	O
address	O
assignment	O
on	O
the	O
receiving	O
interfaces	O
for	O
local	O
or	O
global	O
routing	O
purposes	O
.	O
</s>
