<s>
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
is	O
the	O
most	O
recent	O
version	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
,	O
the	O
communications	O
protocol	O
that	O
provides	O
an	O
identification	O
and	O
location	O
system	O
for	O
computers	O
on	O
networks	O
and	O
routes	O
traffic	O
across	O
the	O
Internet	O
.	O
</s>
<s>
IPv6	B-Protocol
was	O
developed	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
to	O
deal	O
with	O
the	O
long-anticipated	O
problem	O
of	O
IPv4	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
,	O
and	O
is	O
intended	O
to	O
replace	O
IPv4	B-Protocol
.	O
</s>
<s>
In	O
December	O
1998	O
,	O
IPv6	B-Protocol
became	O
a	O
Draft	O
Standard	O
for	O
the	O
IETF	O
,	O
which	O
subsequently	O
ratified	O
it	O
as	O
an	B-Architecture
Internet	I-Architecture
Standard	O
on	O
14	O
July	O
2017	O
.	O
</s>
<s>
Devices	O
on	O
the	O
Internet	O
are	O
assigned	O
a	O
unique	O
IP	B-Protocol
address	I-Protocol
for	O
identification	O
and	O
location	O
definition	O
.	O
</s>
<s>
With	O
the	O
rapid	O
growth	O
of	O
the	O
Internet	O
after	O
commercialization	O
in	O
the	O
1990s	O
,	O
it	O
became	O
evident	O
that	O
far	O
more	O
addresses	O
would	O
be	O
needed	O
to	O
connect	O
devices	O
than	O
the	O
IPv4	B-Protocol
address	I-Protocol
space	O
had	O
available	O
.	O
</s>
<s>
IPv6	B-Protocol
uses	O
128-bit	O
addresses	O
,	O
theoretically	O
allowing	O
2128	O
,	O
or	O
approximately	O
total	O
addresses	O
.	O
</s>
<s>
The	O
two	O
protocols	O
are	O
not	O
designed	O
to	O
be	O
interoperable	O
,	O
and	O
thus	O
direct	O
communication	O
between	O
them	O
is	O
impossible	O
,	O
complicating	O
the	O
move	O
to	O
IPv6	B-Protocol
.	O
</s>
<s>
However	O
,	O
several	O
transition	B-Protocol
mechanisms	I-Protocol
have	O
been	O
devised	O
to	O
rectify	O
this	O
.	O
</s>
<s>
IPv6	B-Protocol
provides	O
other	O
technical	O
benefits	O
in	O
addition	O
to	O
a	O
larger	O
addressing	O
space	O
.	O
</s>
<s>
In	O
particular	O
,	O
it	O
permits	O
hierarchical	O
address	O
allocation	O
methods	O
that	O
facilitate	O
route	B-Protocol
aggregation	I-Protocol
across	O
the	O
Internet	O
,	O
and	O
thus	O
limit	O
the	O
expansion	O
of	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
addresses	I-Protocol
are	O
represented	O
as	O
eight	O
groups	O
of	O
four	O
hexadecimal	O
digits	O
each	O
,	O
separated	O
by	O
colons	O
.	O
</s>
<s>
IPv6	B-Protocol
is	O
an	B-Architecture
Internet	I-Architecture
Layer	O
protocol	O
for	O
packet-switched	B-Protocol
internetworking	B-Architecture
and	O
provides	O
end-to-end	O
datagram	B-Protocol
transmission	O
across	O
multiple	O
IP	O
networks	O
,	O
closely	O
adhering	O
to	O
the	O
design	O
principles	O
developed	O
in	O
the	O
previous	O
version	O
of	O
the	O
protocol	O
,	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
offering	O
more	O
addresses	O
,	O
IPv6	B-Protocol
also	O
implements	O
features	O
not	O
present	O
in	O
IPv4	B-Protocol
.	O
</s>
<s>
It	O
simplifies	O
aspects	O
of	O
address	O
configuration	O
,	O
network	O
renumbering	O
,	O
and	O
router	B-Protocol
announcements	O
when	O
changing	O
network	O
connectivity	O
providers	O
.	O
</s>
<s>
It	O
simplifies	O
processing	O
of	O
packets	B-Protocol
in	O
routers	B-Protocol
by	O
placing	O
the	O
responsibility	O
for	O
packet	B-Protocol
fragmentation	O
into	O
the	O
end	O
points	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
subnet	B-Protocol
size	O
is	O
standardized	O
by	O
fixing	O
the	O
size	O
of	O
the	O
host	O
identifier	O
portion	O
of	O
an	O
address	O
to	O
64	O
bits	O
.	O
</s>
<s>
The	O
addressing	O
architecture	O
of	O
IPv6	B-Protocol
is	O
defined	O
in	O
and	O
allows	O
three	O
different	O
types	O
of	O
transmission	O
:	O
unicast	B-Protocol
,	O
anycast	B-Protocol
and	O
multicast	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
was	O
the	O
first	O
publicly	O
used	O
version	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
IPv4	B-Protocol
was	O
developed	O
as	O
a	O
research	O
project	O
by	O
the	O
Defense	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
DARPA	O
)	O
,	O
a	O
United	O
States	O
Department	O
of	O
Defense	O
agency	O
,	O
before	O
becoming	O
the	O
foundation	O
for	O
the	O
Internet	O
and	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
IPv4	B-Protocol
includes	O
an	O
addressing	O
system	O
that	O
uses	O
numerical	O
identifiers	O
consisting	O
of	O
32	O
bits	O
.	O
</s>
<s>
These	O
addresses	O
are	O
typically	O
displayed	O
in	O
dot-decimal	B-Protocol
notation	I-Protocol
as	O
decimal	O
values	O
of	O
four	O
octets	O
,	O
each	O
in	O
the	O
range	O
0	O
to	O
255	O
,	O
or	O
8	O
bits	O
per	O
number	O
.	O
</s>
<s>
Thus	O
,	O
IPv4	B-Protocol
provides	O
an	O
addressing	O
capability	O
of	O
232	O
or	O
approximately	O
4.3	O
billion	O
addresses	O
.	O
</s>
<s>
Address	O
exhaustion	O
was	O
not	O
initially	O
a	O
concern	O
in	O
IPv4	B-Protocol
as	O
this	O
version	O
was	O
originally	O
presumed	O
to	O
be	O
a	O
test	O
of	O
DARPA	O
's	O
networking	O
concepts	O
.	O
</s>
<s>
In	O
the	O
early	O
1990s	O
,	O
even	O
after	O
the	O
redesign	O
of	O
the	O
addressing	O
system	O
using	O
a	O
classless	B-Protocol
network	I-Protocol
model	O
,	O
it	O
became	O
clear	O
that	O
this	O
would	O
not	O
suffice	O
to	O
prevent	O
IPv4	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
,	O
and	O
that	O
further	O
changes	O
to	O
the	O
Internet	O
infrastructure	O
were	O
needed	O
.	O
</s>
<s>
The	O
last	O
unassigned	O
top-level	O
address	O
blocks	O
of	O
16	O
million	O
IPv4	B-Protocol
addresses	O
were	O
allocated	O
in	O
February	O
2011	O
by	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
to	O
the	O
five	O
regional	O
Internet	O
registries	O
(	O
RIRs	O
)	O
.	O
</s>
<s>
However	O
,	O
each	O
RIR	O
still	O
has	O
available	O
address	O
pools	O
and	O
is	O
expected	O
to	O
continue	O
with	O
standard	O
address	O
allocation	O
policies	O
until	O
one	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
block	O
remains	O
.	O
</s>
<s>
This	O
leaves	O
African	O
Network	O
Information	O
Center	O
(	O
AFRINIC	O
)	O
as	O
the	O
sole	O
regional	O
internet	O
registry	O
that	O
is	O
still	O
using	O
the	O
normal	O
protocol	O
for	O
distributing	O
IPv4	B-Protocol
addresses	O
.	O
</s>
<s>
As	O
of	O
November	O
2018	O
,	O
AFRINIC	O
's	O
minimum	O
allocation	O
is	O
or	O
1024	O
IPv4	B-Protocol
addresses	O
.	O
</s>
<s>
RIPE	O
NCC	O
announced	O
that	O
it	O
had	O
fully	O
run	O
out	O
of	O
IPv4	B-Protocol
addresses	O
on	O
25	O
November	O
2019	O
,	O
and	O
called	O
for	O
greater	O
progress	O
on	O
the	O
adoption	O
of	O
IPv6	B-Protocol
.	O
</s>
<s>
It	O
is	O
widely	O
expected	O
that	O
the	O
Internet	O
will	O
use	O
IPv4	B-Protocol
alongside	O
IPv6	B-Protocol
for	O
the	O
foreseeable	O
future	O
.	O
</s>
<s>
On	O
the	O
Internet	O
,	O
data	O
is	O
transmitted	O
in	O
the	O
form	O
of	O
network	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
specifies	O
a	O
new	O
packet	B-Protocol
format	I-Protocol
,	O
designed	O
to	O
minimize	O
packet	B-General_Concept
header	I-General_Concept
processing	O
by	O
routers	B-Protocol
.	O
</s>
<s>
Because	O
the	O
headers	B-General_Concept
of	O
IPv4	B-Protocol
packets	B-Protocol
and	O
IPv6	B-Protocol
packets	I-Protocol
are	O
significantly	O
different	O
,	O
the	O
two	O
protocols	O
are	O
not	O
interoperable	O
.	O
</s>
<s>
However	O
,	O
most	O
transport	O
and	O
application-layer	B-Application
protocols	O
need	O
little	O
or	O
no	O
change	O
to	O
operate	O
over	O
IPv6	B-Protocol
;	O
exceptions	O
are	O
application	B-Application
protocols	I-Application
that	O
embed	O
Internet-layer	B-Protocol
addresses	O
,	O
such	O
as	O
File	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
FTP	O
)	O
and	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
(	O
NTP	O
)	O
,	O
where	O
the	O
new	O
address	O
format	O
may	O
cause	O
conflicts	O
with	O
existing	O
protocol	O
syntax	O
.	O
</s>
<s>
The	O
main	O
advantage	O
of	O
IPv6	B-Protocol
over	O
IPv4	B-Protocol
is	O
its	O
larger	O
address	O
space	O
.	O
</s>
<s>
The	O
size	O
of	O
an	O
IPv6	B-Protocol
address	I-Protocol
is	O
128	O
bits	O
,	O
compared	O
to	O
32	O
bits	O
in	O
IPv4	B-Protocol
.	O
</s>
<s>
Some	O
blocks	O
of	O
this	O
space	O
and	O
some	O
specific	O
addresses	O
are	O
reserved	B-Protocol
for	I-Protocol
special	I-Protocol
uses	I-Protocol
.	O
</s>
<s>
While	O
this	O
address	O
space	O
is	O
very	O
large	O
,	O
it	O
was	O
not	O
the	O
intent	O
of	O
the	O
designers	O
of	O
IPv6	B-Protocol
to	O
assure	O
geographical	O
saturation	O
with	O
usable	O
addresses	O
.	O
</s>
<s>
Rather	O
,	O
the	O
longer	O
addresses	O
simplify	O
allocation	O
of	O
addresses	O
,	O
enable	O
efficient	O
route	B-Protocol
aggregation	I-Protocol
,	O
and	O
allow	O
implementation	O
of	O
special	O
addressing	O
features	O
.	O
</s>
<s>
In	O
IPv4	B-Protocol
,	O
complex	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
methods	O
were	O
developed	O
to	O
make	O
the	O
best	O
use	O
of	O
the	O
small	O
address	O
space	O
.	O
</s>
<s>
The	O
standard	O
size	O
of	O
a	O
subnet	B-Protocol
in	O
IPv6	B-Protocol
is	O
264	O
addresses	O
,	O
about	O
four	O
billion	O
times	O
the	O
size	O
of	O
the	O
entire	O
IPv4	B-Protocol
address	I-Protocol
space	O
.	O
</s>
<s>
Thus	O
,	O
actual	O
address	O
space	O
utilization	O
will	O
be	O
small	O
in	O
IPv6	B-Protocol
,	O
but	O
network	O
management	O
and	O
routing	O
efficiency	O
are	O
improved	O
by	O
the	O
large	O
subnet	B-Protocol
space	O
and	O
hierarchical	O
route	B-Protocol
aggregation	I-Protocol
.	O
</s>
<s>
Multicasting	O
,	O
the	O
transmission	O
of	O
a	O
packet	B-Protocol
to	O
multiple	O
destinations	O
in	O
a	O
single	O
send	O
operation	O
,	O
is	O
part	O
of	O
the	O
base	O
specification	O
in	O
IPv6	B-Protocol
.	O
</s>
<s>
In	O
IPv4	B-Protocol
this	O
is	O
an	O
optional	O
(	O
although	O
commonly	O
implemented	O
)	O
feature	O
.	O
</s>
<s>
IPv6	B-Protocol
multicast	O
addressing	O
has	O
features	O
and	O
protocols	O
in	O
common	O
with	O
IPv4	B-Protocol
multicast	O
,	O
but	O
also	O
provides	O
changes	O
and	O
improvements	O
by	O
eliminating	O
the	O
need	O
for	O
certain	O
protocols	O
.	O
</s>
<s>
IPv6	B-Protocol
does	O
not	O
implement	O
traditional	O
IP	B-Protocol
broadcast	I-Protocol
,	O
i.e.	O
</s>
<s>
the	O
transmission	O
of	O
a	O
packet	B-Protocol
to	O
all	O
hosts	O
on	O
the	O
attached	O
link	O
using	O
a	O
special	O
broadcast	B-Protocol
address	I-Protocol
,	O
and	O
therefore	O
does	O
not	O
define	O
broadcast	B-Operating_System
addresses	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
the	O
same	O
result	O
is	O
achieved	O
by	O
sending	O
a	O
packet	B-Protocol
to	O
the	O
link-local	O
all	O
nodes	O
multicast	O
group	O
at	O
address	O
ff02::1	O
,	O
which	O
is	O
analogous	O
to	O
IPv4	B-Protocol
multicasting	O
to	O
address	O
224.0.0.1	O
.	O
</s>
<s>
IPv6	B-Protocol
also	O
provides	O
for	O
new	O
multicast	O
implementations	O
,	O
including	O
embedding	O
rendezvous	O
point	O
addresses	O
in	O
an	O
IPv6	B-Protocol
multicast	O
group	O
address	O
,	O
which	O
simplifies	O
the	O
deployment	O
of	O
inter-domain	O
solutions	O
.	O
</s>
<s>
In	O
IPv4	B-Protocol
it	O
is	O
very	O
difficult	O
for	O
an	O
organization	O
to	O
get	O
even	O
one	O
globally	O
routable	O
multicast	O
group	O
assignment	O
,	O
and	O
the	O
implementation	O
of	O
inter-domain	O
solutions	O
is	O
arcane	O
.	O
</s>
<s>
Unicast	B-Protocol
address	I-Protocol
assignments	O
by	O
a	O
local	O
Internet	O
registry	O
for	O
IPv6	B-Protocol
have	O
at	O
least	O
a	O
64-bit	O
routing	O
prefix	O
,	O
yielding	O
the	O
smallest	O
subnet	B-Protocol
size	O
available	O
in	O
IPv6	B-Protocol
(	O
also	O
64	O
bits	O
)	O
.	O
</s>
<s>
With	O
such	O
an	O
assignment	O
it	O
is	O
possible	O
to	O
embed	O
the	O
unicast	B-Protocol
address	I-Protocol
prefix	O
into	O
the	O
IPv6	B-Protocol
multicast	O
address	O
format	O
,	O
while	O
still	O
providing	O
a	O
32-bit	O
block	O
,	O
the	O
least	O
significant	O
bits	O
of	O
the	O
address	O
,	O
or	O
approximately	O
4.2	O
billion	O
multicast	O
group	O
identifiers	O
.	O
</s>
<s>
Thus	O
each	O
user	O
of	O
an	O
IPv6	B-Protocol
subnet	B-Protocol
automatically	O
has	O
available	O
a	O
set	O
of	O
globally	O
routable	O
source-specific	O
multicast	O
groups	O
for	O
multicast	O
applications	O
.	O
</s>
<s>
IPv6	B-Protocol
hosts	O
configure	O
themselves	O
automatically	O
.	O
</s>
<s>
Every	O
interface	O
has	O
a	O
self-generated	O
link-local	B-Protocol
address	I-Protocol
and	O
,	O
when	O
connected	O
to	O
a	O
network	O
,	O
conflict	O
resolution	O
is	O
performed	O
and	O
routers	B-Protocol
provide	O
network	O
prefixes	O
via	O
router	B-Protocol
advertisements	O
.	O
</s>
<s>
Stateless	O
configuration	O
of	O
routers	B-Protocol
can	O
be	O
achieved	O
with	O
a	O
special	O
router	B-Protocol
renumbering	O
protocol	O
.	O
</s>
<s>
When	O
necessary	O
,	O
hosts	O
may	O
configure	O
additional	O
stateful	O
addresses	O
via	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
DHCPv6	B-Protocol
)	O
or	O
static	O
addresses	O
manually	O
.	O
</s>
<s>
Like	O
IPv4	B-Protocol
,	O
IPv6	B-Protocol
supports	O
globally	O
unique	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
design	O
of	O
IPv6	B-Protocol
intended	O
to	O
re-emphasize	O
the	O
end-to-end	B-Application
principle	I-Application
of	O
network	O
design	O
that	O
was	O
originally	O
conceived	O
during	O
the	O
establishment	O
of	O
the	O
early	O
Internet	O
by	O
rendering	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
obsolete	O
.	O
</s>
<s>
A	O
stable	O
,	O
unique	O
,	O
globally	O
addressable	O
IP	B-Protocol
address	I-Protocol
would	O
facilitate	O
tracking	O
a	O
device	O
across	O
networks	O
.	O
</s>
<s>
To	O
address	O
these	O
privacy	O
concerns	O
,	O
the	O
SLAAC	O
protocol	O
includes	O
what	O
are	O
typically	O
called	O
"	O
privacy	O
addresses	O
"	O
or	O
,	O
more	O
correctly	O
,	O
"	O
temporary	O
addresses	O
"	O
,	O
codified	O
in	O
RFC	O
4941	O
,	O
"	O
Privacy	O
Extensions	O
for	O
Stateless	B-Protocol
Address	I-Protocol
Autoconfiguration	I-Protocol
in	O
IPv6	B-Protocol
"	O
.	O
</s>
<s>
Temporary	O
addresses	O
are	O
used	O
by	O
default	O
by	O
Windows	O
since	O
XP	B-Application
SP1	O
,	O
macOS	O
since	O
(	O
Mac	O
OS	O
X	O
)	O
10.7	O
,	O
Android	O
since	O
4.0	O
,	O
and	O
iOS	O
since	O
version	O
4.3	O
.	O
</s>
<s>
Use	O
of	O
temporary	O
addresses	O
by	O
Linux	B-Operating_System
distributions	O
varies	O
.	O
</s>
<s>
Renumbering	O
an	O
existing	O
network	O
for	O
a	O
new	O
connectivity	O
provider	O
with	O
different	O
routing	O
prefixes	O
is	O
a	O
major	O
effort	O
with	O
IPv4	B-Protocol
.	O
</s>
<s>
With	O
IPv6	B-Protocol
,	O
however	O
,	O
changing	O
the	O
prefix	O
announced	O
by	O
a	O
few	O
routers	B-Protocol
can	O
in	O
principle	O
renumber	O
an	O
entire	O
network	O
,	O
since	O
the	O
host	O
identifiers	O
(	O
the	O
least-significant	O
64	O
bits	O
of	O
an	O
address	O
)	O
can	O
be	O
independently	O
self-configured	O
by	O
a	O
host	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
Security	I-Protocol
(	O
IPsec	B-Protocol
)	O
was	O
originally	O
developed	O
for	O
IPv6	B-Protocol
,	O
but	O
found	O
widespread	O
deployment	O
first	O
in	O
IPv4	B-Protocol
,	O
for	O
which	O
it	O
was	O
re-engineered	O
.	O
</s>
<s>
IPsec	B-Protocol
was	O
a	O
mandatory	O
part	O
of	O
all	O
IPv6	B-Protocol
protocol	O
implementations	O
,	O
and	O
Internet	B-Protocol
Key	I-Protocol
Exchange	I-Protocol
(	O
IKE	O
)	O
was	O
recommended	O
,	O
but	O
with	O
RFC	O
6434	O
the	O
inclusion	O
of	O
IPsec	B-Protocol
in	O
IPv6	B-Protocol
implementations	O
was	O
downgraded	O
to	O
a	O
recommendation	O
because	O
it	O
was	O
considered	O
impractical	O
to	O
require	O
full	O
IPsec	B-Protocol
implementation	O
for	O
all	O
types	O
of	O
devices	O
that	O
may	O
use	O
IPv6	B-Protocol
.	O
</s>
<s>
However	O
,	O
as	O
of	O
RFC	O
4301	O
IPv6	B-Protocol
protocol	O
implementations	O
that	O
do	O
implement	O
IPsec	B-Protocol
need	O
to	O
implement	O
IKEv2	B-Protocol
and	O
need	O
to	O
support	O
a	O
minimum	O
set	O
of	O
cryptographic	O
algorithms	O
.	O
</s>
<s>
This	O
requirement	O
will	O
help	O
to	O
make	O
IPsec	B-Protocol
implementations	I-Protocol
more	O
interoperable	O
between	O
devices	O
from	O
different	O
vendors	O
.	O
</s>
<s>
The	O
IPsec	B-Protocol
Authentication	O
Header	B-General_Concept
(	O
AH	O
)	O
and	O
the	O
Encapsulating	O
Security	O
Payload	O
header	B-General_Concept
(	O
ESP	O
)	O
are	O
implemented	O
as	O
IPv6	B-Protocol
extension	O
headers	B-General_Concept
.	O
</s>
<s>
The	O
packet	B-General_Concept
header	I-General_Concept
in	O
IPv6	B-Protocol
is	O
simpler	O
than	O
the	O
IPv4	B-Protocol
header	B-General_Concept
.	O
</s>
<s>
Many	O
rarely	O
used	O
fields	O
have	O
been	O
moved	O
to	O
optional	O
header	B-General_Concept
extensions	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
packet	I-Protocol
header	B-General_Concept
has	O
simplified	O
the	O
process	O
of	O
packet	B-Protocol
forwarding	O
by	O
routers	B-Protocol
.	O
</s>
<s>
Although	O
IPv6	B-Protocol
packet	B-General_Concept
headers	I-General_Concept
are	O
at	O
least	O
twice	O
the	O
size	O
of	O
IPv4	B-Protocol
packet	B-General_Concept
headers	I-General_Concept
,	O
processing	O
of	O
packets	B-Protocol
that	O
only	O
contain	O
the	O
base	O
IPv6	B-Protocol
header	B-General_Concept
by	O
routers	B-Protocol
may	O
,	O
in	O
some	O
cases	O
,	O
be	O
more	O
efficient	O
,	O
because	O
less	O
processing	O
is	O
required	O
in	O
routers	B-Protocol
due	O
to	O
the	O
headers	B-General_Concept
being	O
aligned	O
to	O
match	O
common	O
word	O
sizes	O
.	O
</s>
<s>
However	O
,	O
many	O
devices	O
implement	O
IPv6	B-Protocol
support	O
in	O
software	O
(	O
as	O
opposed	O
to	O
hardware	O
)	O
,	O
thus	O
resulting	O
in	O
very	O
bad	O
packet	B-Protocol
processing	O
performance	O
.	O
</s>
<s>
Additionally	O
,	O
for	O
many	O
implementations	O
,	O
the	O
use	O
of	O
Extension	O
Headers	B-General_Concept
causes	O
packets	B-Protocol
to	O
be	O
processed	O
by	O
a	O
router	B-Protocol
's	O
CPU	O
,	O
leading	O
to	O
poor	O
performance	O
or	O
even	O
security	O
issues	O
.	O
</s>
<s>
Moreover	O
,	O
an	O
IPv6	B-Protocol
header	B-General_Concept
does	O
not	O
include	O
a	O
checksum	O
.	O
</s>
<s>
The	O
IPv4	B-Protocol
header	I-Protocol
checksum	I-Protocol
is	O
calculated	O
for	O
the	O
IPv4	B-Protocol
header	B-General_Concept
,	O
and	O
has	O
to	O
be	O
recalculated	O
by	O
routers	B-Protocol
every	O
time	O
the	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
(	O
called	O
hop	B-Protocol
limit	I-Protocol
in	O
the	O
IPv6	B-Protocol
protocol	O
)	O
is	O
reduced	O
by	O
one	O
.	O
</s>
<s>
The	O
absence	O
of	O
a	O
checksum	O
in	O
the	O
IPv6	B-Protocol
header	B-General_Concept
furthers	O
the	O
end-to-end	B-Application
principle	I-Application
of	O
Internet	O
design	O
,	O
which	O
envisioned	O
that	O
most	O
processing	O
in	O
the	O
network	O
occurs	O
in	O
the	O
leaf	O
nodes	O
.	O
</s>
<s>
Integrity	O
protection	O
for	O
the	O
data	O
that	O
is	O
encapsulated	O
in	O
the	O
IPv6	B-Protocol
packet	I-Protocol
is	O
assumed	O
to	O
be	O
assured	O
by	O
both	O
the	O
link	B-Protocol
layer	I-Protocol
or	O
error	O
detection	O
in	O
higher-layer	O
protocols	O
,	O
namely	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
and	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
on	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
Thus	O
,	O
while	O
IPv4	B-Protocol
allowed	O
UDP	O
datagram	B-Protocol
headers	B-General_Concept
to	O
have	O
no	O
checksum	O
(	O
indicated	O
by	O
0	O
in	O
the	O
header	B-General_Concept
field	O
)	O
,	O
IPv6	B-Protocol
requires	O
a	O
checksum	O
in	O
UDP	O
headers	B-General_Concept
.	O
</s>
<s>
IPv6	B-Protocol
routers	B-Protocol
do	O
not	O
perform	O
IP	B-Protocol
fragmentation	I-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
hosts	O
are	O
required	O
either	O
to	O
perform	O
path	B-Protocol
MTU	I-Protocol
discovery	I-Protocol
,	O
perform	O
end-to-end	O
fragmentation	O
,	O
or	O
send	O
packets	B-Protocol
no	O
larger	O
than	O
the	O
default	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
,	O
which	O
is	O
1280	O
octets	O
.	O
</s>
<s>
Unlike	O
mobile	B-Protocol
IPv4	I-Protocol
,	O
mobile	B-Protocol
IPv6	I-Protocol
avoids	O
triangular	B-Protocol
routing	I-Protocol
and	O
is	O
therefore	O
as	O
efficient	O
as	O
native	O
IPv6	B-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
routers	B-Protocol
may	O
also	O
allow	O
entire	O
subnets	B-Protocol
to	O
move	O
to	O
a	O
new	O
router	B-Protocol
connection	O
point	O
without	O
renumbering	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
packet	I-Protocol
header	B-General_Concept
has	O
a	O
minimum	O
size	O
of	O
40	O
octets	O
(	O
320	O
bits	O
)	O
.	O
</s>
<s>
This	O
provides	O
the	O
opportunity	O
to	O
extend	O
the	O
protocol	O
in	O
the	O
future	O
without	O
affecting	O
the	O
core	O
packet	B-Protocol
structure	O
.	O
</s>
<s>
However	O
,	O
RFC	O
7872	O
notes	O
that	O
some	O
network	O
operators	O
drop	O
IPv6	B-Protocol
packets	I-Protocol
with	O
extension	O
headers	B-General_Concept
when	O
they	O
traverse	O
transit	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
IPv4	B-Protocol
limits	O
packets	B-Protocol
to	O
65,535	O
(	O
216−1	O
)	O
octets	O
of	O
payload	O
.	O
</s>
<s>
An	O
IPv6	B-Protocol
node	O
can	O
optionally	O
handle	O
packets	B-Protocol
over	O
this	O
limit	O
,	O
referred	O
to	O
as	O
jumbograms	B-Protocol
,	O
which	O
can	O
be	O
as	O
large	O
as	O
4,294,967,295	O
(	O
232−1	O
)	O
octets	O
.	O
</s>
<s>
The	O
use	O
of	O
jumbograms	B-Protocol
may	O
improve	O
performance	O
over	O
high-MTU	O
links	O
.	O
</s>
<s>
The	O
use	O
of	O
jumbograms	B-Protocol
is	O
indicated	O
by	O
the	O
Jumbo	B-Protocol
Payload	I-Protocol
Option	O
extension	O
header	B-General_Concept
.	O
</s>
<s>
An	O
IPv6	B-Protocol
packet	I-Protocol
has	O
two	O
parts	O
:	O
a	O
header	B-General_Concept
and	O
payload	O
.	O
</s>
<s>
The	O
header	B-General_Concept
consists	O
of	O
a	O
fixed	O
portion	O
with	O
minimal	O
functionality	O
required	O
for	O
all	O
packets	B-Protocol
and	O
may	O
be	O
followed	O
by	O
optional	O
extensions	O
to	O
implement	O
special	O
features	O
.	O
</s>
<s>
The	O
fixed	O
header	B-General_Concept
occupies	O
the	O
first	O
40octets	O
(	O
320	O
bits	O
)	O
of	O
the	O
IPv6	B-Protocol
packet	I-Protocol
.	O
</s>
<s>
It	O
contains	O
the	O
source	O
and	O
destination	O
addresses	O
,	O
traffic	O
class	O
,	O
hop	O
count	O
,	O
and	O
the	O
type	O
of	O
the	O
optional	O
extension	O
or	O
payload	O
which	O
follows	O
the	O
header	B-General_Concept
.	O
</s>
<s>
This	O
Next	O
Header	B-General_Concept
field	O
tells	O
the	O
receiver	O
how	O
to	O
interpret	O
the	O
data	O
which	O
follows	O
the	O
header	B-General_Concept
.	O
</s>
<s>
If	O
the	O
packet	B-Protocol
contains	O
options	O
,	O
this	O
field	O
contains	O
the	O
option	O
type	O
of	O
the	O
next	O
option	O
.	O
</s>
<s>
The	O
"	O
Next	O
Header	B-General_Concept
"	O
field	O
of	O
the	O
last	O
option	O
points	O
to	O
the	O
upper-layer	O
protocol	O
that	O
is	O
carried	O
in	O
the	O
packet	B-Protocol
's	O
payload	O
.	O
</s>
<s>
The	O
current	O
use	O
of	O
the	O
IPv6	B-Protocol
Traffic	O
Class	O
field	O
divides	O
this	O
between	O
a	O
6	O
bit	O
Differentiated	O
Services	O
Code	O
Point	O
and	O
a	O
2-bit	O
Explicit	O
Congestion	O
Notification	O
field	O
.	O
</s>
<s>
Extension	O
headers	B-General_Concept
carry	O
options	O
that	O
are	O
used	O
for	O
special	O
treatment	O
of	O
a	O
packet	B-Protocol
in	O
the	O
network	O
,	O
e.g.	O
,	O
for	O
routing	O
,	O
fragmentation	O
,	O
and	O
for	O
security	O
using	O
the	O
IPsec	B-Protocol
framework	O
.	O
</s>
<s>
With	O
a	O
Jumbo	B-Protocol
Payload	I-Protocol
option	O
(	O
in	O
a	O
Hop-By-Hop	O
Options	O
extension	O
header	B-General_Concept
)	O
,	O
the	O
payload	O
must	O
be	O
less	O
than	O
4GB	O
.	O
</s>
<s>
Unlike	O
with	O
IPv4	B-Protocol
,	O
routers	B-Protocol
never	O
fragment	O
a	O
packet	B-Protocol
.	O
</s>
<s>
Hosts	O
are	O
expected	O
to	O
use	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
to	O
make	O
their	O
packets	B-Protocol
small	O
enough	O
to	O
reach	O
the	O
destination	O
without	O
needing	O
to	O
be	O
fragmented	O
.	O
</s>
<s>
See	O
IPv6	B-Protocol
packet	I-Protocol
fragmentation	O
.	O
</s>
<s>
IPv6	B-Protocol
addresses	I-Protocol
have	O
128	O
bits	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
IPv6	B-Protocol
address	I-Protocol
space	I-Protocol
implements	O
a	O
different	O
design	O
philosophy	O
than	O
in	O
IPv4	B-Protocol
,	O
in	O
which	O
subnetting	B-Protocol
was	O
used	O
to	O
improve	O
the	O
efficiency	O
of	O
utilization	O
of	O
the	O
small	O
address	O
space	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
the	O
address	O
space	O
is	O
deemed	O
large	O
enough	O
for	O
the	O
foreseeable	O
future	O
,	O
and	O
a	O
local	O
area	O
subnet	B-Protocol
always	O
uses	O
64	O
bits	O
for	O
the	O
host	O
portion	O
of	O
the	O
address	O
,	O
designated	O
as	O
the	O
interface	O
identifier	O
,	O
while	O
the	O
most-significant	O
64	O
bits	O
are	O
used	O
as	O
the	O
routing	O
prefix	O
.	O
</s>
<s>
While	O
the	O
myth	O
has	O
existed	O
regarding	O
IPv6	B-Protocol
subnets	B-Protocol
being	O
impossible	O
to	O
scan	O
,	O
RFC	O
7707	O
notes	O
that	O
patterns	O
resulting	O
from	O
some	O
IPv6	B-Protocol
address	I-Protocol
configuration	O
techniques	O
and	O
algorithms	O
allow	O
address	O
scanning	O
in	O
many	O
real-world	O
scenarios	O
.	O
</s>
<s>
The	O
128	O
bits	O
of	O
an	O
IPv6	B-Protocol
address	I-Protocol
are	O
represented	O
in	O
8	O
groups	O
of	O
16	O
bits	O
each	O
.	O
</s>
<s>
For	O
convenience	O
and	O
clarity	O
,	O
the	O
representation	O
of	O
an	O
IPv6	B-Protocol
address	I-Protocol
may	O
be	O
shortened	O
with	O
the	O
following	O
rules	O
:	O
</s>
<s>
As	O
an	O
IPv6	B-Protocol
address	I-Protocol
may	O
have	O
more	O
than	O
one	O
representation	O
,	O
the	O
IETF	O
has	O
issued	O
a	O
proposed	O
standard	O
for	O
representing	O
them	O
in	O
text	O
.	O
</s>
<s>
Because	O
IPv6	B-Protocol
addresses	I-Protocol
contain	O
colons	O
,	O
and	O
URLs	O
use	O
colons	O
to	O
separate	O
the	O
host	O
from	O
the	O
port	O
number	O
,	O
RFC2732	O
specifies	O
that	O
an	O
IPv6	B-Protocol
address	I-Protocol
used	O
as	O
the	O
host-part	O
of	O
a	O
URL	O
should	O
be	O
enclosed	O
in	O
square	O
brackets	O
,	O
e.g.	O
</s>
<s>
All	O
interfaces	O
of	O
IPv6	B-Protocol
hosts	O
require	O
a	O
link-local	B-Protocol
address	I-Protocol
,	O
which	O
have	O
the	O
prefix	O
.	O
</s>
<s>
This	O
prefix	O
is	O
followed	O
by	O
54	O
bits	O
that	O
can	O
be	O
used	O
for	O
subnetting	B-Protocol
,	O
although	O
they	O
are	O
typically	O
set	O
to	O
zeros	O
,	O
and	O
a	O
64-bit	O
interface	O
identifier	O
.	O
</s>
<s>
The	O
host	O
can	O
compute	O
and	O
assign	O
the	O
Interface	O
identifier	O
by	O
itself	O
without	O
the	O
presence	O
or	O
cooperation	O
of	O
an	O
external	O
network	O
component	O
like	O
a	O
DHCP	O
server	O
,	O
in	O
a	O
process	O
called	O
link-local	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
.	O
</s>
<s>
The	O
lower	O
64	O
bits	O
of	O
the	O
link-local	B-Protocol
address	I-Protocol
(	O
the	O
suffix	O
)	O
were	O
originally	O
derived	O
from	O
the	O
MAC	O
address	O
of	O
the	O
underlying	O
network	O
interface	O
card	O
.	O
</s>
<s>
IPv6	B-Protocol
uses	O
a	O
new	O
mechanism	O
for	O
mapping	O
IP	B-Protocol
addresses	I-Protocol
to	O
link-layer	B-Protocol
addresses	O
(	O
e.g.	O
</s>
<s>
MAC	O
addresses	O
)	O
,	O
because	O
it	O
does	O
not	O
support	O
the	O
broadcast	B-Operating_System
addressing	O
method	O
,	O
on	O
which	O
the	O
functionality	O
of	O
the	O
Address	O
Resolution	O
Protocol	O
(	O
ARP	O
)	O
in	O
IPv4	B-Protocol
is	O
based	O
.	O
</s>
<s>
IPv6	B-Protocol
implements	O
the	O
Neighbor	B-Protocol
Discovery	I-Protocol
Protocol	I-Protocol
(	O
NDP	O
,	O
ND	O
)	O
in	O
the	O
link	B-Protocol
layer	I-Protocol
,	O
which	O
relies	O
on	O
ICMPv6	B-Protocol
and	O
multicast	O
transmission	O
.	O
</s>
<s>
IPv6	B-Protocol
hosts	O
verify	O
the	O
uniqueness	O
of	O
their	O
IPv6	B-Protocol
addresses	I-Protocol
in	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
by	O
sending	O
a	O
neighbor	O
solicitation	O
message	O
asking	O
for	O
the	O
link-layer	B-Protocol
address	O
of	O
the	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
If	O
any	O
other	O
host	O
in	O
the	O
LAN	B-General_Concept
is	O
using	O
that	O
address	O
,	O
it	O
responds	O
.	O
</s>
<s>
A	O
host	O
bringing	O
up	O
a	O
new	O
IPv6	B-Protocol
interface	O
first	O
generates	O
a	O
unique	O
link-local	B-Protocol
address	I-Protocol
using	O
one	O
of	O
several	O
mechanisms	O
designed	O
to	O
generate	O
a	O
unique	O
address	O
.	O
</s>
<s>
Once	O
a	O
unique	O
link-local	B-Protocol
address	I-Protocol
is	O
established	O
,	O
the	O
IPv6	B-Protocol
host	O
determines	O
whether	O
the	O
LAN	B-General_Concept
is	O
connected	O
on	O
this	O
link	O
to	O
any	O
router	B-Protocol
interface	O
that	O
supports	O
IPv6	B-Protocol
.	O
</s>
<s>
It	O
does	O
so	O
by	O
sending	O
out	O
an	O
ICMPv6	B-Protocol
router	B-Protocol
solicitation	O
message	O
to	O
the	O
all-routers	O
multicast	O
group	O
with	O
its	O
link-local	B-Protocol
address	I-Protocol
as	O
source	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
answer	O
after	O
a	O
predetermined	O
number	O
of	O
attempts	O
,	O
the	O
host	O
concludes	O
that	O
no	O
routers	B-Protocol
are	O
connected	O
.	O
</s>
<s>
If	O
it	O
does	O
get	O
a	O
response	O
,	O
known	O
as	O
a	O
router	B-Protocol
advertisement	O
,	O
from	O
a	O
router	B-Protocol
,	O
the	O
response	O
includes	O
the	O
network	O
configuration	O
information	O
to	O
allow	O
establishment	O
of	O
a	O
globally	O
unique	O
address	O
with	O
an	O
appropriate	O
unicast	B-Protocol
network	O
prefix	O
.	O
</s>
<s>
The	O
Manage	O
bit	O
,	O
which	O
indicates	O
whether	O
or	O
not	O
the	O
host	O
should	O
use	O
DHCP	O
to	O
obtain	O
additional	O
addresses	O
rather	O
than	O
rely	O
on	O
an	O
auto-configured	O
address	O
from	O
the	O
router	B-Protocol
advertisement	O
.	O
</s>
<s>
The	O
other	O
information	O
consists	O
of	O
one	O
or	O
more	O
prefix	O
information	O
options	O
for	O
the	O
subnets	B-Protocol
that	O
the	O
host	O
is	O
attached	O
to	O
,	O
a	O
lifetime	O
for	O
the	O
prefix	O
,	O
and	O
two	O
flags	O
:	O
</s>
<s>
On-link	O
:	O
If	O
this	O
flag	O
is	O
set	O
,	O
the	O
host	O
will	O
treat	O
all	O
addresses	O
on	O
the	O
specific	O
subnet	B-Protocol
as	O
being	O
on-link	O
and	O
send	O
packets	B-Protocol
directly	O
to	O
them	O
instead	O
of	O
sending	O
them	O
to	O
a	O
router	B-Protocol
for	O
the	O
duration	O
of	O
the	O
given	O
lifetime	O
.	O
</s>
<s>
The	O
prefix	O
is	O
supplied	O
from	O
router	B-Protocol
advertisements	O
on	O
the	O
network	O
.	O
</s>
<s>
Stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
(	O
SLAAC	O
)	O
requires	O
a	O
address	O
block	O
,	O
as	O
defined	O
in	O
.	O
</s>
<s>
The	O
initial	O
recommendation	O
stated	O
assignment	O
of	O
a	O
subnet	B-Protocol
to	O
end-consumer	O
sites	O
(	O
)	O
.	O
</s>
<s>
In	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
,	O
hostnames	B-Protocol
are	O
mapped	O
to	O
IPv6	B-Protocol
addresses	I-Protocol
by	O
AAAA	B-Protocol
(	O
"	O
quad-A	O
"	O
)	O
resource	O
records	O
.	O
</s>
<s>
For	O
reverse	B-Protocol
resolution	I-Protocol
,	O
the	O
IETF	O
reserved	O
the	O
domain	O
ip6.arpa	O
,	O
where	O
the	O
name	O
space	O
is	O
hierarchically	O
divided	O
by	O
the	O
1-digit	O
hexadecimal	O
representation	O
of	O
nibble	O
units	O
(	O
4	O
bits	O
)	O
of	O
the	O
IPv6	B-Protocol
address	I-Protocol
.	O
</s>
<s>
When	O
a	O
dual-stack	O
host	O
queries	O
a	O
DNS	B-Protocol
server	O
to	O
resolve	O
a	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
(	O
FQDN	B-Protocol
)	O
,	O
the	O
DNS	B-Protocol
client	O
of	O
the	O
host	O
sends	O
two	O
DNS	B-Protocol
requests	O
,	O
one	O
querying	O
A	O
records	O
and	O
the	O
other	O
querying	O
AAAA	B-Protocol
records	I-Protocol
.	O
</s>
<s>
The	O
host	O
operating	B-General_Concept
system	I-General_Concept
may	O
be	O
configured	O
with	O
a	O
preference	O
for	O
address	O
selection	O
rules	O
.	O
</s>
<s>
An	O
alternate	O
record	O
type	O
was	O
used	O
in	O
early	O
DNS	B-Protocol
implementations	O
for	O
IPv6	B-Protocol
,	O
designed	O
to	O
facilitate	O
network	O
renumbering	O
,	O
the	O
A6	O
records	O
for	O
the	O
forward	O
lookup	O
and	O
a	O
number	O
of	O
other	O
innovations	O
such	O
as	O
bit-string	O
labels	O
and	O
DNAME	O
records	O
.	O
</s>
<s>
IPv6	B-Protocol
is	O
not	O
foreseen	O
to	O
supplant	O
IPv4	B-Protocol
instantaneously	O
.	O
</s>
<s>
Therefore	O
,	O
IPv6	B-Protocol
transition	I-Protocol
mechanisms	I-Protocol
are	O
needed	O
to	O
enable	O
IPv6	B-Protocol
hosts	O
to	O
reach	O
IPv4	B-Protocol
services	O
and	O
to	O
allow	O
isolated	O
IPv6	B-Protocol
hosts	O
and	O
networks	O
to	O
reach	O
each	O
other	O
over	O
IPv4	B-Protocol
infrastructure	O
.	O
</s>
<s>
According	O
to	O
Silvia	O
Hagen	O
,	O
a	O
dual-stack	O
implementation	O
of	O
the	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
on	O
devices	O
is	O
the	O
easiest	O
way	O
to	O
migrate	O
to	O
IPv6	B-Protocol
.	O
</s>
<s>
Many	O
other	O
transition	B-Protocol
mechanisms	I-Protocol
use	O
tunneling	O
to	O
encapsulate	O
IPv6	B-Protocol
traffic	O
within	O
IPv4	B-Protocol
networks	I-Protocol
and	O
vice	O
versa	O
.	O
</s>
<s>
This	O
is	O
an	O
imperfect	O
solution	O
,	O
which	O
reduces	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
of	O
a	O
link	O
and	O
therefore	O
complicates	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
,	O
and	O
may	O
increase	O
latency	O
.	O
</s>
<s>
Dual-stack	O
IP	O
implementations	O
provide	O
complete	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
protocol	O
stacks	O
in	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
a	O
computer	O
or	O
network	B-Device
device	I-Device
on	O
top	O
of	O
the	O
common	O
physical	B-Application
layer	I-Application
implementation	O
,	O
such	O
as	O
Ethernet	O
.	O
</s>
<s>
This	O
permits	O
dual-stack	O
hosts	O
to	O
participate	O
in	O
IPv6	B-Protocol
and	O
IPv4	B-Protocol
networks	I-Protocol
simultaneously	O
.	O
</s>
<s>
A	O
device	O
with	O
dual-stack	O
implementation	O
in	O
the	O
operating	B-General_Concept
system	I-General_Concept
has	O
an	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
address	I-Protocol
,	O
and	O
can	O
communicate	O
with	O
other	O
nodes	O
in	O
the	O
LAN	B-General_Concept
or	O
the	O
Internet	O
using	O
either	O
IPv4	B-Protocol
or	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
protocol	O
is	O
used	O
by	O
both	O
IP	B-Protocol
protocols	I-Protocol
to	O
resolve	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
names	I-Protocol
(	O
FQDN	B-Protocol
)	O
and	O
IP	B-Protocol
addresses	I-Protocol
,	O
but	O
dual	O
stack	O
requires	O
that	O
the	O
resolving	O
DNS	B-Protocol
server	O
can	O
resolve	O
both	O
types	O
of	O
addresses	O
.	O
</s>
<s>
Such	O
a	O
dual	O
stack	O
DNS	B-Protocol
server	O
would	O
hold	O
IPv4	B-Protocol
addresses	O
in	O
the	O
A	O
records	O
,	O
and	O
IPv6	B-Protocol
addresses	I-Protocol
in	O
the	O
AAAA	B-Protocol
records	I-Protocol
.	O
</s>
<s>
Depending	O
on	O
the	O
destination	O
that	O
is	O
to	O
be	O
resolved	O
,	O
a	O
DNS	B-Protocol
name	I-Protocol
server	O
may	O
return	O
an	O
IPv4	B-Protocol
or	O
IPv6	B-Protocol
IP	B-Protocol
address	I-Protocol
,	O
or	O
both	O
.	O
</s>
<s>
A	O
default	O
address	O
selection	O
mechanism	O
,	O
or	O
preferred	O
protocol	O
,	O
needs	O
to	O
be	O
configured	O
either	O
on	O
hosts	O
or	O
the	O
DNS	B-Protocol
server	O
.	O
</s>
<s>
The	O
IETF	O
has	O
published	O
Happy	B-Protocol
Eyeballs	I-Protocol
to	O
assist	O
dual	O
stack	O
applications	O
,	O
so	O
that	O
they	O
can	O
connect	O
using	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
,	O
but	O
prefer	O
an	O
IPv6	B-Protocol
connection	O
if	O
it	O
is	O
available	O
.	O
</s>
<s>
However	O
,	O
dual-stack	O
also	O
needs	O
to	O
be	O
implemented	O
on	O
all	O
routers	B-Protocol
between	O
the	O
host	O
and	O
the	O
service	O
for	O
which	O
the	O
DNS	B-Protocol
server	O
has	O
returned	O
an	O
IPv6	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Dual-stack	O
clients	O
should	O
only	O
be	O
configured	O
to	O
prefer	O
IPv6	B-Protocol
,	O
if	O
the	O
network	O
is	O
able	O
to	O
forward	O
IPv6	B-Protocol
packets	I-Protocol
using	O
the	O
IPv6	B-Protocol
versions	O
of	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
When	O
dual	O
stack	O
networks	O
protocols	O
are	O
in	O
place	O
the	O
application	B-Application
layer	I-Application
can	O
be	O
migrated	O
to	O
IPv6	B-Protocol
.	O
</s>
<s>
While	O
dual-stack	O
is	O
supported	O
by	O
major	O
operating	B-General_Concept
system	I-General_Concept
and	O
network	B-Device
device	I-Device
vendors	O
,	O
legacy	O
networking	B-Device
hardware	I-Device
and	O
servers	O
do	O
not	O
support	O
IPv6	B-Protocol
.	O
</s>
<s>
Internet	O
service	O
providers	O
(	O
ISPs	O
)	O
are	O
increasingly	O
providing	O
their	O
business	O
and	O
private	O
customers	O
with	O
public-facing	O
IPv6	B-Protocol
global	O
unicast	B-Protocol
addresses	O
.	O
</s>
<s>
If	O
IPv4	B-Protocol
is	O
still	O
used	O
in	O
the	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
,	O
however	O
,	O
and	O
the	O
ISP	O
can	O
only	O
provide	O
one	O
public-facing	O
IPv6	B-Protocol
address	I-Protocol
,	O
the	O
IPv4	B-Protocol
LAN	B-General_Concept
addresses	O
are	O
translated	O
into	O
the	O
public	O
facing	O
IPv6	B-Protocol
address	I-Protocol
using	O
NAT64	B-Protocol
,	O
a	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
mechanism	O
.	O
</s>
<s>
Some	O
ISPs	O
cannot	O
provide	O
their	O
customers	O
with	O
public-facing	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
addresses	I-Protocol
,	O
thus	O
supporting	O
dual-stack	O
networking	O
,	O
because	O
some	O
ISPs	O
have	O
exhausted	O
their	O
globally	O
routable	O
IPv4	B-Protocol
address	I-Protocol
pool	O
.	O
</s>
<s>
Meanwhile	O
,	O
ISP	O
customers	O
are	O
still	O
trying	O
to	O
reach	O
IPv4	B-Protocol
web	B-Application
servers	I-Application
and	O
other	O
destinations	O
.	O
</s>
<s>
A	O
significant	O
percentage	O
of	O
ISPs	O
in	O
all	O
regional	O
Internet	O
registry	O
(	O
RIR	O
)	O
zones	O
have	O
obtained	O
IPv6	B-Protocol
address	I-Protocol
space	I-Protocol
.	O
</s>
<s>
This	O
includes	O
many	O
of	O
the	O
world	O
's	O
major	O
ISPs	O
and	O
mobile	O
network	O
operators	O
,	O
such	O
as	O
Verizon	B-Application
Wireless	I-Application
,	O
StarHub	O
Cable	O
,	O
Chubu	O
Telecommunications	O
,	O
Kabel	O
Deutschland	O
,	O
Swisscom	O
,	O
T-Mobile	O
,	O
Internode	O
and	O
Telefónica	O
.	O
</s>
<s>
While	O
some	O
ISPs	O
still	O
allocate	O
customers	O
only	O
IPv4	B-Protocol
addresses	O
,	O
many	O
ISPs	O
allocate	O
their	O
customers	O
only	O
an	O
IPv6	B-Protocol
or	O
dual-stack	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
ISPs	O
report	O
the	O
share	O
of	O
IPv6	B-Protocol
traffic	O
from	O
customers	O
over	O
their	O
network	O
to	O
be	O
anything	O
between	O
20%	O
and	O
40%	O
,	O
but	O
by	O
mid-2017	O
IPv6	B-Protocol
traffic	O
still	O
only	O
accounted	O
for	O
a	O
fraction	O
of	O
total	O
traffic	O
at	O
several	O
large	O
Internet	B-Protocol
exchange	I-Protocol
points	I-Protocol
(	O
IXPs	B-Protocol
)	O
.	O
</s>
<s>
A	O
2017	O
survey	O
found	O
that	O
many	O
DSL	O
customers	O
that	O
were	O
served	O
by	O
a	O
dual	O
stack	O
ISP	O
did	O
not	O
request	O
DNS	B-Protocol
servers	O
to	O
resolve	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
names	I-Protocol
into	O
IPv6	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
survey	O
also	O
found	O
that	O
the	O
majority	O
of	O
traffic	O
from	O
IPv6-ready	O
web-server	B-Application
resources	O
were	O
still	O
requested	O
and	O
served	O
over	O
IPv4	B-Protocol
,	O
mostly	O
due	O
to	O
ISP	O
customers	O
that	O
did	O
not	O
use	O
the	O
dual	O
stack	O
facility	O
provided	O
by	O
their	O
ISP	O
and	O
to	O
a	O
lesser	O
extent	O
due	O
to	O
customers	O
of	O
IPv4-only	O
ISPs	O
.	O
</s>
<s>
The	O
technical	O
basis	O
for	O
tunneling	O
,	O
or	O
encapsulating	O
IPv6	B-Protocol
packets	I-Protocol
in	O
IPv4	B-Protocol
packets	B-Protocol
,	O
is	O
outlined	O
in	O
RFC	O
4213	O
.	O
</s>
<s>
When	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
was	O
IPv4-only	O
,	O
one	O
of	O
the	O
frequently	O
used	O
tunneling	O
protocols	O
was	O
6to4	B-Protocol
.	O
</s>
<s>
Teredo	B-Protocol
tunneling	I-Protocol
was	O
also	O
frequently	O
used	O
for	O
integrating	O
IPv6	B-Protocol
LANs	B-General_Concept
with	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
backbone	B-General_Concept
.	O
</s>
<s>
Teredo	O
is	O
outlined	O
in	O
RFC	O
4380	O
and	O
allows	O
IPv6	B-Protocol
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
to	O
tunnel	O
over	O
IPv4	B-Protocol
networks	I-Protocol
,	O
by	O
encapsulating	O
IPv6	B-Protocol
packets	I-Protocol
within	O
UDP	O
.	O
</s>
<s>
The	O
Teredo	O
relay	O
is	O
an	O
IPv6	B-Protocol
router	B-Protocol
that	O
mediates	O
between	O
a	O
Teredo	O
server	O
and	O
the	O
native	O
IPv6	B-Protocol
network	O
.	O
</s>
<s>
It	O
was	O
expected	O
that	O
6to4	B-Protocol
and	O
Teredo	O
would	O
be	O
widely	O
deployed	O
until	O
ISP	O
networks	O
would	O
switch	O
to	O
native	O
IPv6	B-Protocol
,	O
but	O
by	O
2014	O
Google	O
Statistics	O
showed	O
that	O
the	O
use	O
of	O
both	O
mechanisms	O
had	O
dropped	O
to	O
almost	O
0	O
.	O
</s>
<s>
Hybrid	O
dual-stack	O
IPv6/IPv4	O
implementations	O
recognize	O
a	O
special	O
class	O
of	O
addresses	O
,	O
the	O
IPv4-mapped	O
IPv6	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
These	O
addresses	O
are	O
typically	O
written	O
with	O
a	O
96-bit	O
prefix	O
in	O
the	O
standard	O
IPv6	B-Protocol
format	O
,	O
and	O
the	O
remaining	O
32	O
bits	O
are	O
written	O
in	O
the	O
customary	O
dot-decimal	B-Protocol
notation	I-Protocol
of	O
IPv4	B-Protocol
.	O
</s>
<s>
Addresses	O
in	O
this	O
group	O
consist	O
of	O
an	O
80-bit	O
prefix	O
of	O
zeros	O
,	O
the	O
next	O
16	O
bits	O
are	O
ones	O
,	O
and	O
the	O
remaining	O
,	O
least-significant	O
32	O
bits	O
contain	O
the	O
IPv4	B-Protocol
address	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
represents	O
the	O
IPv4	B-Protocol
address	I-Protocol
.	O
</s>
<s>
A	O
previous	O
format	O
,	O
called	O
"	O
IPv4-compatible	O
IPv6	B-Protocol
address	I-Protocol
"	O
,	O
was	O
;	O
however	O
,	O
this	O
method	O
is	O
deprecated	O
.	O
</s>
<s>
Because	O
of	O
the	O
significant	O
internal	O
differences	O
between	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
protocol	O
stacks	O
,	O
some	O
of	O
the	O
lower-level	O
functionality	O
available	O
to	O
programmers	O
in	O
the	O
IPv6	B-Protocol
stack	O
does	O
not	O
work	O
the	O
same	O
when	O
used	O
with	O
IPv4-mapped	O
addresses	O
.	O
</s>
<s>
Some	O
common	O
IPv6	B-Protocol
stacks	O
do	O
not	O
implement	O
the	O
IPv4-mapped	O
address	O
feature	O
,	O
either	O
because	O
the	O
IPv6	B-Protocol
and	O
IPv4	B-Protocol
stacks	O
are	O
separate	O
implementations	O
(	O
e.g.	O
,	O
Microsoft	B-Application
Windows	I-Application
2000	O
,	O
XP	B-Application
,	O
and	O
Server	O
2003	O
)	O
,	O
or	O
because	O
of	O
security	O
concerns	O
(	O
OpenBSD	B-Operating_System
)	O
.	O
</s>
<s>
On	O
these	O
operating	B-General_Concept
systems	I-General_Concept
,	O
a	O
program	O
must	O
open	O
a	O
separate	O
socket	O
for	O
each	O
IP	B-Protocol
protocol	I-Protocol
it	O
uses	O
.	O
</s>
<s>
On	O
some	O
systems	O
,	O
e.g.	O
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
NetBSD	B-Device
,	O
and	O
FreeBSD	B-Operating_System
,	O
this	O
feature	O
is	O
controlled	O
by	O
the	O
socket	O
option	O
IPV6_V6ONLY	O
.	O
</s>
<s>
The	O
address	B-Protocol
prefix	I-Protocol
is	O
a	O
class	O
of	O
IPv4-embedded	B-Protocol
IPv6	I-Protocol
addresses	I-Protocol
for	O
use	O
in	O
NAT64	B-Protocol
transition	O
methods	O
.	O
</s>
<s>
For	O
example	O
,	O
represents	O
the	O
IPv4	B-Protocol
address	I-Protocol
.	O
</s>
<s>
A	O
number	O
of	O
security	O
implications	O
may	O
arise	O
from	O
the	O
use	O
of	O
IPv6	B-Protocol
.	O
</s>
<s>
Some	O
of	O
them	O
may	O
be	O
related	O
with	O
the	O
IPv6	B-Protocol
protocols	O
themselves	O
,	O
while	O
others	O
may	O
be	O
related	O
with	O
implementation	O
flaws	O
.	O
</s>
<s>
The	O
addition	O
of	O
nodes	O
having	O
IPv6	B-Protocol
enabled	O
by	O
default	O
by	O
the	O
software	O
manufacturer	O
may	O
result	O
in	O
the	O
inadvertent	O
creation	O
of	O
shadow	O
networks	O
,	O
causing	O
IPv6	B-Protocol
traffic	O
flowing	O
into	O
networks	O
having	O
only	O
IPv4	B-Protocol
security	O
management	O
in	O
place	O
.	O
</s>
<s>
This	O
may	O
also	O
occur	O
with	O
operating	B-General_Concept
system	I-General_Concept
upgrades	O
,	O
when	O
the	O
newer	O
operating	B-General_Concept
system	I-General_Concept
enables	O
IPv6	B-Protocol
by	O
default	O
,	O
while	O
the	O
older	O
one	O
did	O
not	O
.	O
</s>
<s>
Failing	O
to	O
update	O
the	O
security	O
infrastructure	O
to	O
accommodate	O
IPv6	B-Protocol
can	O
lead	O
to	O
IPv6	B-Protocol
traffic	O
bypassing	O
it	O
.	O
</s>
<s>
Shadow	O
networks	O
have	O
occurred	O
on	O
business	O
networks	O
in	O
which	O
enterprises	O
are	O
replacing	O
Windows	B-Application
XP	I-Application
systems	O
that	O
do	O
not	O
have	O
an	O
IPv6	B-Protocol
stack	O
enabled	O
by	O
default	O
,	O
with	O
Windows	B-Device
7	I-Device
systems	O
,	O
that	O
do	O
.	O
</s>
<s>
Some	O
IPv6	B-Protocol
stack	O
implementors	O
have	O
therefore	O
recommended	O
disabling	O
IPv4	B-Protocol
mapped	O
addresses	O
and	O
instead	O
using	O
a	O
dual-stack	O
network	O
where	O
supporting	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
is	O
necessary	O
.	O
</s>
<s>
Research	O
has	O
shown	O
that	O
the	O
use	O
of	O
fragmentation	O
can	O
be	O
leveraged	O
to	O
evade	O
network	O
security	O
controls	O
,	O
similar	O
to	O
IPv4	B-Protocol
.	O
</s>
<s>
As	O
a	O
result	O
,	O
requires	O
that	O
the	O
first	O
fragment	O
of	O
an	O
IPv6	B-Protocol
packet	I-Protocol
contains	O
the	O
entire	O
IPv6	B-Protocol
header	B-General_Concept
chain	O
,	O
such	O
that	O
some	O
very	O
pathological	O
fragmentation	O
cases	O
are	O
forbidden	O
.	O
</s>
<s>
Additionally	O
,	O
as	O
a	O
result	O
of	O
research	O
on	O
the	O
evasion	O
of	O
RA-Guard	O
in	O
,	O
has	O
deprecated	O
the	O
use	O
of	O
fragmentation	O
with	O
Neighbor	B-Protocol
Discovery	I-Protocol
,	O
and	O
discouraged	O
the	O
use	O
of	O
fragmentation	O
with	O
Secure	O
Neighbor	B-Protocol
Discovery	I-Protocol
(	O
SEND	O
)	O
.	O
</s>
<s>
Due	O
to	O
the	O
anticipated	O
global	O
growth	O
of	O
the	O
Internet	O
,	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
in	O
the	O
early	O
1990s	O
started	O
an	O
effort	O
to	O
develop	O
a	O
next	O
generation	O
IP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
In	O
September	O
1993	O
,	O
the	O
IETF	O
created	O
a	O
temporary	O
,	O
ad	O
hoc	O
IP	B-Protocol
Next	I-Protocol
Generation	I-Protocol
(	O
IPng	B-Protocol
)	O
area	O
to	O
deal	O
specifically	O
with	O
such	O
issues	O
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
adopted	O
the	O
IPng	B-Protocol
model	O
on	O
25	O
July	O
1994	O
,	O
with	O
the	O
formation	O
of	O
several	O
IPng	B-Protocol
working	O
groups	O
.	O
</s>
<s>
By	O
1996	O
,	O
a	O
series	O
of	O
RFCs	O
was	O
released	O
defining	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
,	O
starting	O
with	O
.	O
</s>
<s>
(	O
Version	O
5	O
was	O
used	O
by	O
the	O
experimental	O
Internet	B-Protocol
Stream	I-Protocol
Protocol	I-Protocol
.	O
)	O
</s>
<s>
The	O
first	O
RFC	O
to	O
standardize	O
IPv6	B-Protocol
was	O
the	O
in	O
1995	O
,	O
which	O
became	O
obsoleted	O
by	O
in	O
1998	O
.	O
</s>
<s>
In	O
July	O
2017	O
this	O
RFC	O
was	O
superseded	O
by	O
,	O
which	O
elevated	O
IPv6	B-Protocol
to	O
"	O
Internet	O
Standard	O
"	O
(	O
the	O
highest	O
maturity	O
level	O
for	O
IETF	O
protocols	O
)	O
.	O
</s>
<s>
The	O
1993	O
introduction	O
of	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
in	O
the	O
routing	O
and	O
IP	B-Protocol
address	I-Protocol
allocation	O
for	O
the	O
Internet	O
,	O
and	O
the	O
extensive	O
use	O
of	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
,	O
delayed	O
IPv4	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
to	O
allow	O
for	O
IPv6	B-Protocol
deployment	O
,	O
which	O
began	O
in	O
the	O
mid-2000s	O
.	O
</s>
<s>
Universities	O
were	O
among	O
the	O
early	O
adopters	O
of	O
IPv6	B-Protocol
.	O
</s>
<s>
Virginia	O
Tech	O
deployed	O
IPv6	B-Protocol
at	O
a	O
trial	O
location	O
in	O
2004	O
and	O
later	O
expanded	O
IPv6	B-Protocol
deployment	O
across	O
the	O
campus	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
By	O
2016	O
,	O
82%	O
of	O
the	O
traffic	O
on	O
their	O
network	O
used	O
IPv6	B-Protocol
.	O
</s>
<s>
Imperial	O
College	O
London	O
began	O
experimental	O
IPv6	B-Protocol
deployment	O
in	O
2003	O
and	O
by	O
2016	O
the	O
IPv6	B-Protocol
traffic	O
on	O
their	O
networks	O
averaged	O
between	O
20%	O
and	O
40%	O
.	O
</s>
<s>
A	O
significant	O
portion	O
of	O
this	O
IPv6	B-Protocol
traffic	O
was	O
generated	O
through	O
their	O
high	O
energy	O
physics	O
collaboration	O
with	O
CERN	O
,	O
which	O
relies	O
entirely	O
on	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
has	O
supported	O
IPv6	B-Protocol
since	O
2008	O
.	O
</s>
<s>
In	O
the	O
same	O
year	O
,	O
IPv6	B-Protocol
was	O
first	O
used	O
in	O
a	O
major	O
world	O
event	O
during	O
the	O
Beijing	O
2008	O
Summer	O
Olympics	O
.	O
</s>
<s>
By	O
2011	O
,	O
all	O
major	O
operating	B-General_Concept
systems	I-General_Concept
in	O
use	O
on	O
personal	O
computers	O
and	O
server	O
systems	O
had	O
production-quality	O
IPv6	B-Protocol
implementations	O
.	O
</s>
<s>
Cellular	O
telephone	O
systems	O
presented	O
a	O
large	O
deployment	O
field	O
for	O
Internet	B-Protocol
Protocol	I-Protocol
devices	O
as	O
mobile	O
telephone	O
service	O
made	O
the	O
transition	O
from	O
3G	B-Architecture
to	O
4G	B-Architecture
technologies	O
,	O
in	O
which	O
voice	O
is	O
provisioned	O
as	O
a	O
voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
service	O
that	O
would	O
leverage	O
IPv6	B-Protocol
enhancements	O
.	O
</s>
<s>
In	O
2009	O
,	O
the	O
US	O
cellular	O
operator	O
Verizon	B-Application
released	O
technical	O
specifications	O
for	O
devices	O
to	O
operate	O
on	O
its	O
"	O
next-generation	O
"	O
networks	O
.	O
</s>
<s>
The	O
specification	O
mandated	O
IPv6	B-Protocol
operation	O
according	O
to	O
the	O
3GPP	O
Release	O
8	O
Specifications	O
(	O
March	O
2009	O
)	O
,	O
and	O
deprecated	O
IPv4	B-Protocol
as	O
an	O
optional	O
capability	O
.	O
</s>
<s>
The	O
deployment	O
of	O
IPv6	B-Protocol
in	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
continued	O
.	O
</s>
<s>
In	O
2018	O
only	O
25.3	O
%	O
of	O
the	O
about	O
54,000	O
autonomous	B-Architecture
systems	I-Architecture
advertised	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
prefixes	O
in	O
the	O
global	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
routing	O
database	O
.	O
</s>
<s>
A	O
further	O
243	O
networks	O
advertised	O
only	O
an	O
IPv6	B-Protocol
prefix	O
.	O
</s>
<s>
Internet	B-General_Concept
backbone	I-General_Concept
transit	O
networks	O
offering	O
IPv6	B-Protocol
support	O
existed	O
in	O
every	O
country	O
globally	O
,	O
except	O
in	O
parts	O
of	O
Africa	O
,	O
the	O
Middle	O
East	O
and	O
China	O
.	O
</s>
<s>
By	O
mid-2018	O
some	O
major	O
European	O
broadband	B-General_Concept
ISPs	O
had	O
deployed	O
IPv6	B-Protocol
for	O
the	O
majority	O
of	O
their	O
customers	O
.	O
</s>
<s>
Sky	O
UK	O
provided	O
over	O
86%	O
of	O
its	O
customers	O
with	O
IPv6	B-Protocol
,	O
Deutsche	O
Telekom	O
had	O
56%	O
deployment	O
of	O
IPv6	B-Protocol
,	O
XS4ALL	O
in	O
the	O
Netherlands	O
had	O
73%	O
deployment	O
and	O
in	O
Belgium	O
the	O
broadband	B-General_Concept
ISPs	O
VOO	O
and	O
Telenet	O
had	O
73%	O
and	O
63%	O
IPv6	B-Protocol
deployment	O
respectively	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
the	O
broadband	B-General_Concept
ISP	O
Xfinity	O
had	O
an	O
IPv6	B-Protocol
deployment	O
of	O
about	O
66%	O
.	O
</s>
<s>
In	O
2018	O
Xfinity	O
reported	O
an	O
estimated	O
36.1	O
million	O
IPv6	B-Protocol
users	O
,	O
while	O
AT&T	O
reported	O
22.3	O
million	O
IPv6	B-Protocol
users	O
.	O
</s>
