<s>
A	O
subnetwork	B-Protocol
or	O
subnet	B-Protocol
is	O
a	O
logical	O
subdivision	O
of	O
an	O
IP	B-Protocol
network	I-Protocol
.	O
</s>
<s>
The	O
practice	O
of	O
dividing	O
a	O
network	O
into	O
two	O
or	O
more	O
networks	O
is	O
called	O
subnetting	B-Protocol
.	O
</s>
<s>
Computers	O
that	O
belong	O
to	O
the	O
same	O
subnet	B-Protocol
are	O
addressed	O
with	O
an	O
identical	O
most-significant	O
bit-group	O
in	O
their	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
This	O
results	O
in	O
the	O
logical	O
division	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
into	O
two	O
fields	O
:	O
the	O
network	O
number	O
or	O
routing	B-Protocol
prefix	O
and	O
the	O
rest	O
field	O
or	O
host	B-Device
identifier	O
.	O
</s>
<s>
The	O
rest	O
field	O
is	O
an	O
identifier	O
for	O
a	O
specific	O
host	B-Device
or	O
network	O
interface	O
.	O
</s>
<s>
The	O
routing	B-Protocol
prefix	O
may	O
be	O
expressed	O
in	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
notation	O
written	O
as	O
the	O
first	O
address	O
of	O
a	O
network	O
,	O
followed	O
by	O
a	O
slash	O
character	O
(	O
/	O
)	O
,	O
and	O
ending	O
with	O
the	O
bit-length	O
of	O
the	O
prefix	O
.	O
</s>
<s>
For	O
example	O
,	O
is	O
the	O
prefix	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
network	O
starting	O
at	O
the	O
given	O
address	O
,	O
having	O
24	O
bits	O
allocated	O
for	O
the	O
network	O
prefix	O
,	O
and	O
the	O
remaining	O
8	O
bits	O
reserved	O
for	O
host	B-Device
addressing	O
.	O
</s>
<s>
Addresses	O
in	O
the	O
range	O
to	O
belong	O
to	O
this	O
network	O
,	O
with	O
as	O
the	O
subnet	B-Protocol
broadcast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
IPv6	B-Protocol
address	I-Protocol
specification	O
is	O
a	O
large	O
address	O
block	O
with	O
296	O
addresses	O
,	O
having	O
a	O
32-bit	O
routing	B-Protocol
prefix	O
.	O
</s>
<s>
For	O
IPv4	B-Protocol
,	O
a	O
network	O
may	O
also	O
be	O
characterized	O
by	O
its	O
subnet	B-Protocol
mask	I-Protocol
or	O
netmask	B-Protocol
,	O
which	O
is	O
the	O
bitmask	O
that	O
,	O
when	O
applied	O
by	O
a	O
bitwise	O
AND	O
operation	O
to	O
any	O
IP	B-Protocol
address	I-Protocol
in	O
the	O
network	O
,	O
yields	O
the	O
routing	B-Protocol
prefix	O
.	O
</s>
<s>
Subnet	B-Protocol
masks	I-Protocol
are	O
also	O
expressed	O
in	O
dot-decimal	B-Protocol
notation	I-Protocol
like	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
the	O
prefix	O
would	O
have	O
the	O
subnet	B-Protocol
mask	I-Protocol
.	O
</s>
<s>
Traffic	O
is	O
exchanged	O
between	O
subnetworks	B-Protocol
through	O
routers	B-Protocol
when	O
the	O
routing	B-Protocol
prefixes	O
of	O
the	O
source	O
address	O
and	O
the	O
destination	O
address	O
differ	O
.	O
</s>
<s>
A	O
router	B-Protocol
serves	O
as	O
a	O
logical	O
or	O
physical	O
boundary	O
between	O
the	O
subnets	B-Protocol
.	O
</s>
<s>
The	O
benefits	O
of	O
subnetting	B-Protocol
an	O
existing	O
network	O
vary	O
with	O
each	O
deployment	O
scenario	O
.	O
</s>
<s>
In	O
the	O
address	O
allocation	O
architecture	O
of	O
the	O
Internet	O
using	O
CIDR	B-Protocol
and	O
in	O
large	O
organizations	O
,	O
efficient	O
allocation	O
of	O
address	O
space	O
is	O
necessary	O
.	O
</s>
<s>
Subnetting	B-Protocol
may	O
also	O
enhance	O
routing	B-Protocol
efficiency	O
,	O
or	O
have	O
advantages	O
in	O
network	O
management	O
when	O
subnetworks	B-Protocol
are	O
administratively	O
controlled	O
by	O
different	O
entities	O
in	O
a	O
larger	O
organization	O
.	O
</s>
<s>
Subnets	B-Protocol
may	O
be	O
arranged	O
logically	O
in	O
a	O
hierarchical	O
architecture	O
,	O
partitioning	O
an	O
organization	O
's	O
network	B-Protocol
address	I-Protocol
space	O
into	O
a	O
tree-like	O
routing	B-Protocol
structure	O
,	O
or	O
other	O
structures	O
such	O
as	O
meshes	O
.	O
</s>
<s>
Computers	O
participating	O
in	O
a	O
network	O
such	O
as	O
the	O
Internet	O
each	O
have	O
at	O
least	O
one	O
network	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Usually	O
,	O
this	O
address	O
is	O
unique	O
to	O
each	O
device	O
and	O
can	O
either	O
be	O
configured	O
automatically	O
with	O
the	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
(	O
DHCP	B-Protocol
)	O
by	O
a	O
network	O
server	O
,	O
manually	O
by	O
an	O
administrator	O
,	O
or	O
automatically	O
by	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
.	O
</s>
<s>
An	O
address	O
fulfills	O
the	O
functions	O
of	O
identifying	O
the	O
host	B-Device
and	O
locating	O
it	O
on	O
the	O
network	O
.	O
</s>
<s>
The	O
most	O
common	O
network	O
addressing	O
architecture	O
is	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
,	O
but	O
its	O
successor	O
,	O
IPv6	B-Protocol
,	O
has	O
been	O
increasingly	O
deployed	B-Architecture
since	O
approximately	O
2006	O
.	O
</s>
<s>
An	O
IPv4	B-Protocol
address	I-Protocol
consists	O
of	O
32	O
bits	O
.	O
</s>
<s>
An	O
IPv6	B-Protocol
address	I-Protocol
consists	O
of	O
128	O
bits	O
.	O
</s>
<s>
In	O
both	O
systems	O
,	O
an	O
IP	B-Protocol
address	I-Protocol
is	O
divided	O
into	O
two	O
logical	O
parts	O
,	O
the	O
network	O
prefix	O
and	O
the	O
host	B-Device
identifier	O
.	O
</s>
<s>
All	O
hosts	O
on	O
a	O
subnetwork	B-Protocol
have	O
the	O
same	O
network	O
prefix	O
.	O
</s>
<s>
The	O
number	O
of	O
bits	O
allocated	O
within	O
a	O
network	O
to	O
the	O
prefix	O
may	O
vary	O
between	O
subnets	B-Protocol
,	O
depending	O
on	O
the	O
network	O
architecture	O
.	O
</s>
<s>
The	O
host	B-Device
identifier	O
is	O
a	O
unique	O
local	O
identification	O
and	O
is	O
either	O
a	O
host	B-Device
number	O
on	O
the	O
local	O
network	O
or	O
an	O
interface	O
identifier	O
.	O
</s>
<s>
This	O
addressing	O
structure	O
permits	O
the	O
selective	O
routing	B-Protocol
of	O
IP	O
packets	O
across	O
multiple	O
networks	O
via	O
special	O
gateway	O
computers	O
,	O
called	O
routers	B-Protocol
,	O
to	O
a	O
destination	O
host	B-Device
if	O
the	O
network	O
prefixes	O
of	O
origination	O
and	O
destination	O
hosts	O
differ	O
,	O
or	O
sent	O
directly	O
to	O
a	O
target	O
host	B-Device
on	O
the	O
local	O
network	O
if	O
they	O
are	O
the	O
same	O
.	O
</s>
<s>
Routers	B-Protocol
constitute	O
logical	O
or	O
physical	O
borders	O
between	O
the	O
subnets	B-Protocol
,	O
and	O
manage	O
traffic	O
between	O
them	O
.	O
</s>
<s>
Each	O
subnet	B-Protocol
is	O
served	O
by	O
a	O
designated	O
default	O
router	B-Protocol
but	O
may	O
consist	O
internally	O
of	O
multiple	O
physical	O
Ethernet	O
segments	O
interconnected	O
by	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
The	O
routing	B-Protocol
prefix	O
of	O
an	O
address	O
is	O
identified	O
by	O
the	O
subnet	B-Protocol
mask	I-Protocol
,	O
written	O
in	O
the	O
same	O
form	O
used	O
for	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
the	O
subnet	B-Protocol
mask	I-Protocol
for	O
a	O
routing	B-Protocol
prefix	O
that	O
is	O
composed	O
of	O
the	O
most-significant	O
24	O
bits	O
of	O
an	O
IPv4	B-Protocol
address	I-Protocol
is	O
written	O
as	O
.	O
</s>
<s>
The	O
modern	O
standard	O
form	O
of	O
specification	O
of	O
the	O
network	O
prefix	O
is	O
CIDR	B-Protocol
notation	O
,	O
used	O
for	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
This	O
notation	O
was	O
introduced	O
with	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
this	O
is	O
the	O
only	O
standards-based	O
form	O
to	O
denote	O
network	O
or	O
routing	B-Protocol
prefixes	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
IPv4	B-Protocol
network	I-Protocol
with	O
the	O
subnet	B-Protocol
mask	I-Protocol
is	O
written	O
as	O
,	O
and	O
the	O
IPv6	B-Protocol
notation	O
designates	O
the	O
address	O
and	O
its	O
network	O
prefix	O
consisting	O
of	O
the	O
most	O
significant	O
32	O
bits	O
.	O
</s>
<s>
In	O
classful	B-Protocol
networking	O
in	O
IPv4	B-Protocol
,	O
before	O
the	O
introduction	O
of	O
CIDR	B-Protocol
,	O
the	O
network	O
prefix	O
could	O
be	O
directly	O
obtained	O
from	O
the	O
IP	B-Protocol
address	I-Protocol
,	O
based	O
on	O
its	O
highest-order	O
bit	O
sequence	O
.	O
</s>
<s>
This	O
determined	O
the	O
class	O
(	O
A	O
,	O
B	O
,	O
C	O
)	O
of	O
the	O
address	O
and	O
therefore	O
the	O
subnet	B-Protocol
mask	I-Protocol
.	O
</s>
<s>
Since	O
the	O
introduction	O
of	O
CIDR	B-Protocol
,	O
however	O
,	O
the	O
assignment	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
to	O
a	O
network	O
interface	O
requires	O
two	O
parameters	O
,	O
the	O
address	O
and	O
a	O
subnet	B-Protocol
mask	I-Protocol
.	O
</s>
<s>
Given	O
an	O
IPv4	B-Protocol
source	O
address	O
,	O
its	O
associated	O
subnet	B-Protocol
mask	I-Protocol
,	O
and	O
the	O
destination	O
address	O
,	O
a	O
router	B-Protocol
can	O
determine	O
whether	O
the	O
destination	O
is	O
on	O
a	O
locally	O
connected	O
network	O
or	O
a	O
remote	O
network	O
.	O
</s>
<s>
The	O
subnet	B-Protocol
mask	I-Protocol
of	O
the	O
destination	O
is	O
not	O
needed	O
,	O
and	O
is	O
generally	O
not	O
known	O
to	O
a	O
router	B-Protocol
.	O
</s>
<s>
For	O
IPv6	B-Protocol
,	O
however	O
,	O
on-link	O
determination	O
is	O
different	O
in	O
detail	O
and	O
requires	O
the	O
Neighbor	B-Protocol
Discovery	I-Protocol
Protocol	I-Protocol
(	O
NDP	O
)	O
.	O
</s>
<s>
IPv6	B-Protocol
address	I-Protocol
assignment	O
to	O
an	O
interface	O
carries	O
no	O
requirement	O
of	O
a	O
matching	O
on-link	O
prefix	O
and	O
vice	O
versa	O
,	O
with	O
the	O
exception	O
of	O
link-local	O
addresses	O
.	O
</s>
<s>
Since	O
each	O
locally	O
connected	O
subnet	B-Protocol
must	O
be	O
represented	O
by	O
a	O
separate	O
entry	O
in	O
the	O
routing	B-Protocol
tables	I-Protocol
of	O
each	O
connected	O
router	B-Protocol
,	O
subnetting	B-Protocol
increases	O
routing	B-Protocol
complexity	O
.	O
</s>
<s>
However	O
,	O
by	O
careful	O
design	O
of	O
the	O
network	O
,	O
routes	O
to	O
collections	O
of	O
more	O
distant	O
subnets	B-Protocol
within	O
the	O
branches	O
of	O
a	O
tree	O
hierarchy	O
can	O
be	O
aggregated	O
into	O
a	O
supernetwork	B-Protocol
and	O
represented	O
by	O
single	O
routes	O
.	O
</s>
<s>
An	O
IPv4	B-Protocol
subnet	B-Protocol
mask	I-Protocol
consists	O
of	O
32	O
bits	O
;	O
it	O
is	O
a	O
sequence	O
of	O
ones	O
(	O
1	O
)	O
followed	O
by	O
a	O
block	O
of	O
zeros	O
(	O
0	O
)	O
.	O
</s>
<s>
The	O
ones	O
indicate	O
bits	O
in	O
the	O
address	O
used	O
for	O
the	O
network	O
prefix	O
and	O
the	O
trailing	O
block	O
of	O
zeros	O
designates	O
that	O
part	O
as	O
being	O
the	O
host	B-Device
identifier	O
.	O
</s>
<s>
The	O
following	O
example	O
shows	O
the	O
separation	O
of	O
the	O
network	O
prefix	O
and	O
the	O
host	B-Device
identifier	O
from	O
an	O
address	O
(	O
)	O
and	O
its	O
associated	O
subnet	B-Protocol
mask	I-Protocol
(	O
)	O
.	O
</s>
<s>
The	O
result	O
of	O
the	O
bitwise	O
AND	O
operation	O
of	O
IP	B-Protocol
address	I-Protocol
and	O
the	O
subnet	B-Protocol
mask	I-Protocol
is	O
the	O
network	O
prefix	O
.	O
</s>
<s>
The	O
host	B-Device
part	O
,	O
which	O
is	O
,	O
is	O
derived	O
by	O
the	O
bitwise	O
AND	O
operation	O
of	O
the	O
address	O
and	O
the	O
one	O
's	O
complement	O
of	O
the	O
subnet	B-Protocol
mask	I-Protocol
.	O
</s>
<s>
Subnetting	B-Protocol
is	O
the	O
process	O
of	O
designating	O
some	O
high-order	O
bits	O
from	O
the	O
host	B-Device
part	O
as	O
part	O
of	O
the	O
network	O
prefix	O
and	O
adjusting	O
the	O
subnet	B-Protocol
mask	I-Protocol
appropriately	O
.	O
</s>
<s>
This	O
divides	O
a	O
network	O
into	O
smaller	O
subnets	B-Protocol
.	O
</s>
<s>
The	O
following	O
diagram	O
modifies	O
the	O
above	O
example	O
by	O
moving	O
2	O
bits	O
from	O
the	O
host	B-Device
part	O
to	O
the	O
network	O
prefix	O
to	O
form	O
four	O
smaller	O
subnets	B-Protocol
each	O
one	O
quarter	O
of	O
the	O
previous	O
size	O
.	O
</s>
<s>
IPv4	B-Protocol
uses	O
specially	O
designated	O
address	O
formats	O
to	O
facilitate	O
recognition	O
of	O
special	O
address	O
functionality	O
.	O
</s>
<s>
The	O
first	O
and	O
the	O
last	O
subnets	B-Protocol
obtained	O
by	O
subnetting	B-Protocol
a	O
larger	O
network	O
have	O
traditionally	O
had	O
a	O
special	O
designation	O
and	O
,	O
early	O
on	O
,	O
special	O
usage	O
implications	O
.	O
</s>
<s>
In	O
addition	O
,	O
IPv4	B-Protocol
uses	O
the	O
all	O
ones	O
host	B-Protocol
address	I-Protocol
,	O
i.e.	O
</s>
<s>
The	O
first	O
subnet	B-Protocol
obtained	O
from	O
subnetting	B-Protocol
a	O
larger	O
network	O
has	O
all	O
bits	O
in	O
the	O
subnet	B-Protocol
bit	O
group	O
set	O
to	O
zero	O
.	O
</s>
<s>
It	O
is	O
therefore	O
called	O
subnet	B-Protocol
zero	O
.	O
</s>
<s>
The	O
last	O
subnet	B-Protocol
obtained	O
from	O
subnetting	B-Protocol
a	O
larger	O
network	O
has	O
all	O
bits	O
in	O
the	O
subnet	B-Protocol
bit	O
group	O
set	O
to	O
one	O
.	O
</s>
<s>
It	O
is	O
therefore	O
called	O
the	O
all-ones	O
subnet	B-Protocol
.	O
</s>
<s>
The	O
IETF	O
originally	O
discouraged	O
the	O
production	O
use	O
of	O
these	O
two	O
subnets	B-Protocol
.	O
</s>
<s>
When	O
the	O
prefix	O
length	O
is	O
not	O
available	O
,	O
the	O
larger	O
network	O
and	O
the	O
first	O
subnet	B-Protocol
have	O
the	O
same	O
address	O
,	O
which	O
may	O
lead	O
to	O
confusion	O
.	O
</s>
<s>
Similar	O
confusion	O
is	O
possible	O
with	O
the	O
broadcast	B-Protocol
address	I-Protocol
at	O
the	O
end	O
of	O
the	O
last	O
subnet	B-Protocol
.	O
</s>
<s>
Therefore	O
,	O
reserving	O
the	O
subnet	B-Protocol
values	O
consisting	O
of	O
all	O
zeros	O
and	O
all	O
ones	O
on	O
the	O
public	O
Internet	O
was	O
recommended	O
,	O
reducing	O
the	O
number	O
of	O
available	O
subnets	B-Protocol
by	O
two	O
for	O
each	O
subnetting	B-Protocol
.	O
</s>
<s>
Although	O
the	O
all-zeros	O
and	O
the	O
all-ones	O
host	B-Device
values	O
are	O
reserved	O
for	O
the	O
network	B-Protocol
address	I-Protocol
of	O
the	O
subnet	B-Protocol
and	O
its	O
broadcast	B-Protocol
address	I-Protocol
,	O
respectively	O
,	O
in	O
systems	O
using	O
CIDR	B-Protocol
all	O
subnets	B-Protocol
are	O
available	O
in	O
a	O
subdivided	O
network	O
.	O
</s>
<s>
Each	O
broadcast	B-Protocol
address	I-Protocol
,	O
i.e.	O
</s>
<s>
,	O
,	O
…,	O
,	O
reduces	O
only	O
the	O
host	B-Device
count	O
in	O
each	O
subnetwork	B-Protocol
.	O
</s>
<s>
The	O
number	O
of	O
subnetworks	B-Protocol
available	O
and	O
the	O
number	O
of	O
possible	O
hosts	O
in	O
a	O
network	O
may	O
be	O
readily	O
calculated	O
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
network	O
may	O
be	O
subdivided	O
into	O
the	O
following	O
four	O
subnets	B-Protocol
.	O
</s>
<s>
The	O
remaining	O
bits	O
after	O
the	O
subnet	B-Protocol
bits	O
are	O
used	O
for	O
addressing	O
hosts	O
within	O
the	O
subnet	B-Protocol
.	O
</s>
<s>
In	O
the	O
above	O
example	O
,	O
the	O
subnet	B-Protocol
mask	I-Protocol
consists	O
of	O
26	O
bits	O
,	O
making	O
it	O
255.255.255.192	O
,	O
leaving	O
6	O
bits	O
for	O
the	O
host	B-Device
identifier	O
.	O
</s>
<s>
This	O
allows	O
for	O
62	O
host	B-Device
combinations	O
(	O
26−2	O
)	O
.	O
</s>
<s>
In	O
general	O
,	O
the	O
number	O
of	O
available	O
hosts	O
on	O
a	O
subnet	B-Protocol
is	O
2h−2	O
,	O
where	O
h	O
is	O
the	O
number	O
of	O
bits	O
used	O
for	O
the	O
host	B-Device
portion	O
of	O
the	O
address	O
.	O
</s>
<s>
The	O
number	O
of	O
available	O
subnets	B-Protocol
is	O
2n	O
,	O
where	O
n	O
is	O
the	O
number	O
of	O
bits	O
used	O
for	O
the	O
network	O
portion	O
of	O
the	O
address	O
.	O
</s>
<s>
There	O
is	O
an	O
exception	O
to	O
this	O
rule	O
for	O
31-bit	O
subnet	B-Protocol
masks	I-Protocol
,	O
which	O
means	O
the	O
host	B-Device
identifier	O
is	O
only	O
one	O
bit	O
long	O
for	O
two	O
permissible	O
addresses	O
.	O
</s>
<s>
In	O
such	O
networks	O
,	O
usually	O
point-to-point	B-Architecture
links	I-Architecture
,	O
only	O
two	O
hosts	O
(	O
the	O
endpoints	O
)	O
may	O
be	O
connected	O
and	O
a	O
specification	O
of	O
network	O
and	O
broadcast	O
addresses	O
is	O
not	O
necessary	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
IPv6	B-Protocol
address	I-Protocol
space	I-Protocol
differs	O
significantly	O
from	O
IPv4	B-Protocol
.	O
</s>
<s>
The	O
primary	O
reason	O
for	O
subnetting	B-Protocol
in	O
IPv4	B-Protocol
is	O
to	O
improve	O
efficiency	O
in	O
the	O
utilization	O
of	O
the	O
relatively	O
small	O
address	O
space	O
available	O
,	O
particularly	O
to	O
enterprises	O
.	O
</s>
<s>
No	O
such	O
limitations	O
exist	O
in	O
IPv6	B-Protocol
,	O
as	O
the	O
large	O
address	O
space	O
available	O
,	O
even	O
to	O
end-users	O
,	O
is	O
not	O
a	O
limiting	O
factor	O
.	O
</s>
<s>
As	O
in	O
IPv4	B-Protocol
,	O
subnetting	B-Protocol
in	O
IPv6	B-Protocol
is	O
based	O
on	O
the	O
concepts	O
of	O
variable-length	B-Protocol
subnet	I-Protocol
masking	I-Protocol
(	O
VLSM	O
)	O
and	O
the	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
methodology	O
.	O
</s>
<s>
It	O
is	O
used	O
to	O
route	O
traffic	O
between	O
the	O
global	O
allocation	O
spaces	O
and	O
within	O
customer	O
networks	O
between	O
subnets	B-Protocol
and	O
the	O
Internet	O
at	O
large	O
.	O
</s>
<s>
A	O
compliant	O
IPv6	B-Protocol
subnet	B-Protocol
always	O
uses	O
addresses	O
with	O
64	O
bits	O
in	O
the	O
host	B-Device
identifier	O
.	O
</s>
<s>
Given	O
the	O
address	O
size	O
of	O
128	O
bits	O
,	O
it	O
therefore	O
has	O
a	O
/64	B-Protocol
routing	B-Protocol
prefix	O
.	O
</s>
<s>
Although	O
it	O
is	O
technically	O
possible	O
to	O
use	O
smaller	O
subnets	B-Protocol
,	O
they	O
are	O
impractical	O
for	O
local	O
area	O
networks	O
based	O
on	O
Ethernet	O
technology	O
,	O
because	O
64	O
bits	O
are	O
required	O
for	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
recommends	O
the	O
use	O
of	O
subnets	B-Protocol
for	O
point-to-point	B-Architecture
links	I-Architecture
,	O
which	O
have	O
only	O
two	O
hosts	O
.	O
</s>
<s>
IPv6	B-Protocol
does	O
not	O
implement	O
special	O
address	O
formats	O
for	O
broadcast	O
traffic	O
or	O
network	O
numbers	O
,	O
and	O
thus	O
all	O
addresses	O
in	O
a	O
subnet	B-Protocol
are	O
acceptable	O
for	O
host	B-Device
addressing	O
.	O
</s>
<s>
The	O
all-zeroes	O
address	O
is	O
reserved	O
as	O
the	O
subnet-router	O
anycast	O
address	O
.	O
</s>
<s>
The	O
subnet	B-Protocol
router	B-Protocol
anycast	O
address	O
is	O
the	O
lowest	O
address	O
in	O
the	O
subnet	B-Protocol
,	O
so	O
it	O
looks	O
like	O
the	O
“	O
network	B-Protocol
address	I-Protocol
”	O
.	O
</s>
<s>
If	O
a	O
router	B-Protocol
has	O
multiple	O
subnets	B-Protocol
on	O
the	O
same	O
link	O
,	O
then	O
it	O
has	O
multiple	O
subnet	B-Protocol
router	B-Protocol
anycast	O
addresses	O
on	O
that	O
link	O
.	O
</s>
<s>
The	O
first	O
and	O
last	O
address	O
in	O
any	O
network	O
or	O
subnet	B-Protocol
is	O
not	O
allowed	O
to	O
be	O
assigned	O
to	O
any	O
individual	O
host	B-Device
.	O
</s>
<s>
In	O
the	O
past	O
,	O
the	O
recommended	O
allocation	O
for	O
an	O
IPv6	B-Protocol
customer	O
site	O
was	O
an	O
address	O
space	O
with	O
a	O
48-bit	O
(	O
)	O
prefix	O
.	O
</s>
