<s>
A	O
unique	B-Protocol
local	I-Protocol
address	I-Protocol
(	O
ULA	O
)	O
is	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
address	B-Protocol
in	O
the	O
address	B-Protocol
range	O
.	O
</s>
<s>
These	O
addresses	O
are	O
non-globally	O
reachable	O
(	O
routable	O
only	O
within	O
the	O
scope	O
of	O
private	B-Protocol
networks	I-Protocol
,	O
but	O
not	O
the	O
global	O
IPv6	B-Protocol
Internet	I-Protocol
)	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
ULAs	O
are	O
somewhat	O
analogous	O
to	O
IPv4	O
private	B-Protocol
network	I-Protocol
addressing	O
,	O
but	O
with	O
significant	O
differences	O
.	O
</s>
<s>
In	O
December	O
1995	O
,	O
the	O
IPv6	B-Protocol
address	I-Protocol
block	O
was	O
reserved	O
for	O
site-local	O
addresses	O
,	O
that	O
could	O
be	O
used	O
within	O
a	O
"	O
site	O
"	O
for	O
private	O
IPv6	B-Protocol
networks	O
.	O
</s>
<s>
In	O
September	O
2004	O
,	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
deprecated	O
the	O
definition	O
of	O
this	O
address	B-Protocol
range	O
,	O
and	O
postulated	O
solutions	O
to	O
its	O
problems	O
.	O
</s>
<s>
In	O
October	O
2005	O
,	O
the	O
IETF	O
reserved	O
the	O
address	B-Protocol
block	O
for	O
use	O
in	O
private	O
IPv6	B-Protocol
networks	O
and	O
defined	O
the	O
associated	O
term	O
unique	O
local	O
addresses	O
.	O
</s>
<s>
The	O
first	O
bit	O
following	O
the	O
prefix	O
indicates	O
,	O
if	O
set	O
,	O
that	O
the	O
address	B-Protocol
is	O
locally	O
assigned	O
.	O
</s>
<s>
This	O
splits	O
the	O
address	B-Protocol
block	O
in	O
two	O
equally	O
sized	O
halves	O
,	O
and	O
.	O
</s>
<s>
With	O
this	O
prefix	O
,	O
subnets	O
of	O
size	O
are	O
available	O
for	O
the	O
private	B-Protocol
network	I-Protocol
:	O
to	O
.	O
</s>
<s>
Prefixes	O
in	O
the	O
range	O
have	O
some	O
characteristics	O
in	O
common	O
with	O
the	O
IPv4	O
private	B-Protocol
address	I-Protocol
ranges	O
:	O
They	O
are	O
not	O
allocated	O
by	O
an	O
address	B-Protocol
registry	O
and	O
may	O
be	O
used	O
in	O
networks	O
by	O
anyone	O
without	O
outside	O
involvement	O
.	O
</s>
<s>
Reverse	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
entries	O
(	O
in	O
ip6.arpa	O
)	O
for	O
ULAs	O
cannot	O
be	O
delegated	O
in	O
the	O
global	O
DNS	B-Protocol
.	O
</s>
<s>
However	O
,	O
if	O
networks	O
require	O
routing	O
ULAs	O
between	O
each	O
other	O
in	O
the	O
event	O
of	O
a	O
merger	O
,	O
for	O
example	O
,	O
the	O
risk	O
of	O
address	B-Protocol
collision	O
is	O
very	O
small	O
if	O
the	O
RFC	O
4193	O
selection	O
algorithm	O
was	O
used	O
.	O
</s>
<s>
One	O
such	O
example	O
is	O
Amazon	B-Application
Web	I-Application
Services	I-Application
.	O
</s>
<s>
In	O
particular	O
it	O
uses	O
the	O
block	O
for	O
local	O
services	O
such	O
as	O
time	O
sync	O
services	O
or	O
DNS	B-Protocol
resolvers	O
.	O
</s>
