<s>
6to4	B-Protocol
is	O
an	O
Internet	O
transition	O
mechanism	O
for	O
migrating	O
from	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
to	O
version	B-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
and	O
a	O
system	O
that	O
allows	O
IPv6	B-Protocol
packets	I-Protocol
to	O
be	O
transmitted	O
over	O
an	O
IPv4	B-Protocol
network	I-Protocol
(	O
generally	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
)	O
without	O
the	O
need	O
to	O
configure	O
explicit	O
tunnels	O
.	O
</s>
<s>
Special	O
relay	O
servers	O
are	O
also	O
in	O
place	O
that	O
allow	O
6to4	B-Protocol
networks	O
to	O
communicate	O
with	O
native	O
IPv6	B-Protocol
networks	O
.	O
</s>
<s>
6to4	B-Protocol
is	O
especially	O
relevant	O
during	O
the	O
initial	O
phases	O
of	O
deployment	O
to	O
full	O
,	O
native	O
IPv6	B-Protocol
connectivity	O
,	O
since	O
IPv6	B-Protocol
is	O
not	O
required	O
on	O
nodes	O
between	O
the	O
host	O
and	O
the	O
destination	O
.	O
</s>
<s>
6to4	B-Protocol
may	O
be	O
used	O
by	O
an	O
individual	O
host	O
,	O
or	O
by	O
a	O
local	O
IPv6	B-Protocol
network	O
.	O
</s>
<s>
When	O
used	O
by	O
a	O
host	O
,	O
it	O
must	O
have	O
a	O
global	O
IPv4	B-Protocol
address	O
connected	O
,	O
and	O
the	O
host	O
is	O
responsible	O
for	O
encapsulation	B-Protocol
of	O
outgoing	O
IPv6	B-Protocol
packets	I-Protocol
and	O
decapsulation	O
of	O
incoming	O
6to4	B-Protocol
packets	O
.	O
</s>
<s>
If	O
the	O
host	O
is	O
configured	O
to	O
forward	O
packets	O
for	O
other	O
clients	O
,	O
often	O
a	O
local	O
network	O
,	O
it	O
is	O
then	O
a	O
router	B-Protocol
.	O
</s>
<s>
Most	O
IPv6	B-Protocol
networks	O
use	O
autoconfiguration	O
,	O
which	O
requires	O
the	O
last	O
64	O
bits	O
of	O
the	O
address	O
for	O
the	O
host	O
.	O
</s>
<s>
The	O
first	O
64	O
bits	O
are	O
the	O
IPv6	B-Protocol
prefix	O
.	O
</s>
<s>
The	O
first	O
16	O
bits	O
of	O
the	O
prefix	O
are	O
always	O
:	O
,	O
the	O
next	O
32	O
bits	O
are	O
the	O
IPv4	B-Protocol
address	O
,	O
and	O
the	O
last	O
16	O
bits	O
of	O
the	O
prefix	O
are	O
available	O
for	O
addressing	O
multiple	O
IPv6	B-Protocol
subnets	O
behind	O
the	O
same	O
6to4	B-Protocol
router	B-Protocol
.	O
</s>
<s>
Since	O
the	O
IPv6	B-Protocol
hosts	O
using	O
autoconfiguration	O
already	O
have	O
determined	O
the	O
unique	O
64	O
bit	O
host	O
portion	O
of	O
their	O
address	O
,	O
they	O
must	O
simply	O
wait	O
for	O
a	O
Router	B-Protocol
Advertisement	O
indicating	O
the	O
first	O
64	O
bits	O
of	O
prefix	O
to	O
have	O
a	O
complete	O
IPv6	B-Protocol
address	O
.	O
</s>
<s>
A	O
6to4	B-Protocol
router	B-Protocol
will	O
know	O
to	O
send	O
an	O
encapsulated	O
packet	O
directly	O
over	O
IPv4	B-Protocol
if	O
the	O
first	O
16	O
bits	O
are	O
,	O
using	O
the	O
next	O
32	O
as	O
the	O
destination	O
,	O
or	O
otherwise	O
send	O
the	O
packet	O
to	O
a	O
well-known	O
relay	O
server	O
,	O
which	O
has	O
access	O
to	O
native	O
IPv6	B-Protocol
.	O
</s>
<s>
6to4	B-Protocol
does	O
not	O
facilitate	O
interoperation	O
between	O
IPv4-only	O
hosts	O
and	O
IPv6-only	O
hosts	O
(	O
for	O
that	O
,	O
see	O
NAT64	B-Protocol
)	O
.	O
</s>
<s>
6to4	B-Protocol
is	O
simply	O
a	O
transparent	O
mechanism	O
used	O
as	O
a	O
transport	O
layer	O
between	O
IPv6	B-Protocol
nodes	O
.	O
</s>
<s>
Due	O
to	O
the	O
high	O
levels	O
of	O
misconfigured	O
hosts	O
and	O
poor	O
performance	O
observed	O
,	O
an	O
advisory	O
about	O
how	O
6to4	B-Protocol
should	O
be	O
deployed	O
was	O
published	O
in	O
August	O
2011	O
.	O
</s>
<s>
Due	O
to	O
unsolvable	O
operational	O
problems	O
using	O
the	O
6to4	B-Protocol
anycast	B-Protocol
prefix	O
,	O
that	O
part	O
of	O
the	O
standard	O
was	O
deprecated	O
in	O
2015	O
.	O
</s>
<s>
6to4	B-Protocol
performs	O
three	O
functions	O
:	O
</s>
<s>
Assigns	O
a	O
block	O
of	O
IPv6	B-Protocol
address	O
space	O
to	O
any	O
host	O
or	O
network	O
that	O
has	O
a	O
global	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
Encapsulates	O
IPv6	B-Protocol
packets	I-Protocol
inside	O
IPv4	B-Protocol
packets	O
for	O
transmission	O
over	O
an	O
IPv4	B-Protocol
network	I-Protocol
using	O
6in4	B-Protocol
.	O
</s>
<s>
Routes	O
traffic	O
between	O
6to4	B-Protocol
and	O
"	O
native	O
"	O
IPv6	B-Protocol
networks	O
.	O
</s>
<s>
For	O
any	O
32-bit	O
global	O
IPv4	B-Protocol
address	O
that	O
is	O
assigned	O
to	O
a	O
host	O
,	O
a	O
48-bit	O
6to4	B-Protocol
IPv6	B-Protocol
prefix	O
can	O
be	O
constructed	O
for	O
use	O
by	O
that	O
host	O
(	O
and	O
if	O
applicable	O
the	O
network	O
behind	O
it	O
)	O
by	O
appending	O
the	O
IPv4	B-Protocol
address	O
to	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
global	O
IPv4	B-Protocol
address	O
has	O
the	O
corresponding	O
6to4	B-Protocol
prefix	O
.	O
</s>
<s>
Any	O
IPv6	B-Protocol
address	O
that	O
begins	O
with	O
the	O
prefix	O
(	O
in	O
other	O
words	O
,	O
any	O
address	O
with	O
the	O
first	O
two	O
octets	O
of	O
2002	O
hexadecimal	O
)	O
is	O
known	O
as	O
a	O
6to4	B-Protocol
address	I-Protocol
,	O
as	O
opposed	O
to	O
a	O
native	O
IPv6	B-Protocol
address	O
which	O
does	O
not	O
systematically	O
use	O
transition	O
technologies	O
.	O
</s>
<s>
Note	O
that	O
using	O
a	O
reserved	O
IPv4	B-Protocol
address	O
,	O
such	O
as	O
those	O
provided	O
by	O
,	O
is	O
undefined	O
,	O
since	O
these	O
networks	O
are	O
disallowed	O
from	O
being	O
routed	O
on	O
the	O
public	O
Internet	O
.	O
</s>
<s>
For	O
example	O
,	O
using	O
as	O
the	O
router	B-Protocol
's	O
WAN	O
address	O
would	O
be	O
invalid	O
since	O
a	O
return	O
packet	O
would	O
not	O
be	O
able	O
to	O
determine	O
the	O
destination	O
IPv4	B-Protocol
address	O
of	O
the	O
actual	O
sender	O
.	O
</s>
<s>
6to4	B-Protocol
embeds	O
an	O
IPv6	B-Protocol
packet	I-Protocol
in	O
the	O
payload	O
portion	O
of	O
an	O
IPv4	B-Protocol
packet	O
with	O
protocol	O
type	O
41	O
.	O
</s>
<s>
To	O
send	O
an	O
IPv6	B-Protocol
packet	I-Protocol
over	O
an	O
IPv4	B-Protocol
network	I-Protocol
to	O
a	O
6to4	B-Protocol
destination	O
address	O
,	O
an	O
IPv4	B-Protocol
header	O
with	O
protocol	O
type	O
41	O
is	O
prepended	O
to	O
the	O
IPv6	B-Protocol
packet	I-Protocol
.	O
</s>
<s>
The	O
IPv4	B-Protocol
destination	O
address	O
for	O
the	O
prepended	O
packet	O
header	O
is	O
derived	O
from	O
the	O
IPv6	B-Protocol
destination	O
address	O
of	O
the	O
inner	O
packet	O
(	O
which	O
is	O
in	O
the	O
format	O
of	O
a	O
6to4	B-Protocol
address	I-Protocol
)	O
,	O
by	O
extracting	O
the	O
32	O
bits	O
immediately	O
following	O
the	O
IPv6	B-Protocol
destination	O
address	O
's	O
prefix	O
.	O
</s>
<s>
The	O
IPv4	B-Protocol
source	O
address	O
in	O
the	O
prepended	O
packet	O
header	O
is	O
the	O
IPv4	B-Protocol
address	O
of	O
the	O
host	O
or	O
router	B-Protocol
which	O
is	O
sending	O
the	O
packet	O
over	O
IPv4	B-Protocol
.	O
</s>
<s>
The	O
resulting	O
IPv4	B-Protocol
packet	O
is	O
then	O
routed	O
to	O
its	O
IPv4	B-Protocol
destination	O
address	O
just	O
like	O
any	O
other	O
IPv4	B-Protocol
packet	O
.	O
</s>
<s>
To	O
allow	O
hosts	O
and	O
networks	O
using	O
6to4	B-Protocol
addresses	O
to	O
exchange	O
traffic	O
with	O
hosts	O
using	O
"	O
native	O
"	O
IPv6	B-Protocol
addresses	O
,	O
"	O
relay	O
routers	B-Protocol
"	O
have	O
been	O
established	O
.	O
</s>
<s>
A	O
relay	O
router	B-Protocol
connects	O
to	O
an	O
IPv4	B-Protocol
network	I-Protocol
and	O
an	O
IPv6	B-Protocol
network	O
.	O
</s>
<s>
6to4	B-Protocol
packets	O
arriving	O
on	O
an	O
IPv4	B-Protocol
interface	O
will	O
have	O
their	O
IPv6	B-Protocol
payloads	O
routed	O
to	O
the	O
IPv6	B-Protocol
network	O
,	O
while	O
packets	O
arriving	O
on	O
the	O
IPv6	B-Protocol
interface	O
with	O
a	O
destination	O
address	O
prefix	O
of	O
will	O
be	O
encapsulated	O
and	O
forwarded	O
over	O
the	O
IPv4	B-Protocol
network	I-Protocol
.	O
</s>
<s>
There	O
is	O
a	O
difference	O
between	O
a	O
"	O
relay	O
router	B-Protocol
"	O
and	O
a	O
"	O
border	O
router	B-Protocol
"	O
(	O
also	O
known	O
as	O
a	O
"	O
6to4	B-Protocol
border	O
router	B-Protocol
"	O
)	O
.	O
</s>
<s>
A	O
6to4	B-Protocol
border	O
router	B-Protocol
is	O
an	O
IPv6	B-Protocol
router	B-Protocol
supporting	O
a	O
6to4	B-Protocol
pseudo-interface	O
.	O
</s>
<s>
It	O
is	O
normally	O
the	O
border	O
router	B-Protocol
between	O
an	O
IPv6	B-Protocol
site	O
and	O
a	O
wide-area	O
IPv4	B-Protocol
network	I-Protocol
,	O
where	O
the	O
IPv6	B-Protocol
site	O
uses	O
co-related	O
to	O
the	O
IPv4	B-Protocol
address	O
used	O
later	O
on	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
a	O
"	O
relay	O
router	B-Protocol
"	O
is	O
a	O
6to4	B-Protocol
router	B-Protocol
configured	O
to	O
support	O
transit	O
routing	O
between	O
6to4	B-Protocol
addresses	O
and	O
pure	O
native	O
IPv6	B-Protocol
addresses	O
.	O
</s>
<s>
To	O
allow	O
a	O
6to4	B-Protocol
host	O
to	O
communicate	O
with	O
the	O
native	O
IPv6	B-Protocol
Internet	I-Protocol
,	O
it	O
must	O
have	O
its	O
IPv6	B-Protocol
default	B-Protocol
gateway	I-Protocol
set	O
to	O
a	O
6to4	B-Protocol
address	I-Protocol
which	O
contains	O
the	O
IPv4	B-Protocol
address	O
of	O
a	O
6to4	B-Protocol
relay	O
router	B-Protocol
.	O
</s>
<s>
To	O
avoid	O
the	O
need	O
for	O
users	O
to	O
set	O
this	O
up	O
manually	O
,	O
the	O
anycast	B-Protocol
address	O
used	O
to	O
be	O
allocated	O
.	O
</s>
<s>
It	O
could	O
be	O
assigned	O
to	O
any	O
number	O
of	O
6to4	B-Protocol
relay	O
routers	B-Protocol
,	O
so	O
that	O
IPv4	B-Protocol
routing	O
would	O
take	O
care	O
of	O
routing	O
the	O
encapsulated	O
IPv6	B-Protocol
packets	I-Protocol
to	O
the	O
one	O
closest-by	O
for	O
forwarding	O
onto	O
IPv6	B-Protocol
Internet	I-Protocol
.	O
</s>
<s>
Packets	O
from	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
to	O
6to4	B-Protocol
systems	O
must	O
be	O
sent	O
to	O
a	O
6to4	B-Protocol
relay	O
router	B-Protocol
by	O
normal	O
IPv6	B-Protocol
routing	O
methods	O
.	O
</s>
<s>
The	O
specification	O
states	O
that	O
such	O
relay	O
routers	B-Protocol
must	O
only	O
advertise	O
and	O
not	O
subdivisions	O
of	O
it	O
to	O
prevent	O
IPv4	B-Protocol
routes	O
polluting	O
the	O
routing	O
tables	O
of	O
IPv6	B-Protocol
routers	B-Protocol
.	O
</s>
<s>
From	O
here	O
they	O
can	O
then	O
be	O
sent	O
over	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
to	O
the	O
destination	O
.	O
</s>
<s>
For	O
a	O
6to4	B-Protocol
host	O
to	O
have	O
fast	O
and	O
reliable	O
connectivity	O
with	O
a	O
host	O
natively	O
using	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
,	O
both	O
the	O
6to4	B-Protocol
host	O
and	O
the	O
native	O
IPv6	B-Protocol
host	O
must	O
have	O
a	O
route	O
to	O
a	O
fast	O
,	O
reliable	O
and	O
correctly	O
configured	O
relay	O
server	O
.	O
</s>
<s>
The	O
6to4	B-Protocol
host	O
's	O
ISP	O
can	O
ensure	O
that	O
outgoing	O
packets	O
go	O
to	O
such	O
a	O
relay	O
,	O
but	O
they	O
have	O
no	O
control	O
over	O
the	O
relay	O
used	O
for	O
the	O
responses	O
from	O
the	O
native	O
IPv6	B-Protocol
host	O
.	O
</s>
<s>
A	O
variant	O
called	O
IPv6	B-Protocol
rapid	I-Protocol
deployment	I-Protocol
(	O
"	O
6rd	B-Protocol
"	O
)	O
uses	O
the	O
same	O
basic	O
principles	O
as	O
6to4	B-Protocol
but	O
uses	O
a	O
relay	O
operated	O
by	O
the	O
6rd	B-Protocol
user	O
's	O
ISP	O
for	O
traffic	O
in	O
both	O
directions	O
.	O
</s>
<s>
When	O
a	O
site	O
using	O
6to4	B-Protocol
has	O
a	O
fixed	O
global	O
IPv4	B-Protocol
address	O
,	O
its	O
6to4	B-Protocol
IPv6	B-Protocol
prefix	O
is	O
also	O
fixed	O
.	O
</s>
<s>
It	O
is	O
then	O
possible	O
to	O
request	O
reverse	B-Protocol
DNS	I-Protocol
delegation	O
for	O
an	O
individual	O
6to4	B-Protocol
48-bits	O
prefix	O
inside	O
the	O
2.0.0.2.ip6.arpa	O
DNS	B-Protocol
zone	I-Protocol
from	O
the	O
Number	O
Resource	O
Organization	O
at	O
.	O
</s>
<s>
According	O
to	O
,	O
6to4	B-Protocol
routers	B-Protocol
and	O
relays	O
should	O
ensure	O
that	O
:	O
</s>
<s>
either	O
or	O
both	O
the	O
source	O
and	O
destination	O
addresses	O
of	O
any	O
encapsulated	O
packet	O
is	O
within	O
the	O
6to4	B-Protocol
IPv6	B-Protocol
prefix	O
,	O
</s>
<s>
if	O
the	O
source	O
IPv6	B-Protocol
address	O
is	O
a	O
6to4	B-Protocol
IPv6	B-Protocol
address	O
,	O
its	O
corresponding	O
6to4	B-Protocol
router	B-Protocol
IPv4	B-Protocol
address	O
matches	O
the	O
IPv4	B-Protocol
source	O
address	O
in	O
the	O
IPv4	B-Protocol
encapsulation	B-Protocol
header	O
,	O
</s>
<s>
similarly	O
,	O
if	O
the	O
destination	O
IPv6	B-Protocol
address	O
is	O
a	O
6to4	B-Protocol
IPv6	B-Protocol
address	O
,	O
its	O
corresponding	O
6to4	B-Protocol
router	B-Protocol
IPv4	B-Protocol
address	O
matches	O
the	O
IPv4	B-Protocol
destination	O
address	O
in	O
the	O
IPv4	B-Protocol
encapsulation	B-Protocol
header	O
,	O
</s>
<s>
any	O
embedded	O
6to4	B-Protocol
router	B-Protocol
IPv4	B-Protocol
address	O
is	O
global	O
unicast	B-Protocol
.	O
</s>
<s>
6to4	B-Protocol
has	O
been	O
default	O
disabled	O
since	O
the	O
Anniversary	O
Update	O
.	O
</s>
