<s>
Classless	O
Inter-Domain	O
Routing	O
(	O
CIDR	O
)	O
is	O
a	O
method	O
for	O
allocating	O
IP	B-Protocol
addresses	I-Protocol
and	O
for	O
IP	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
introduced	O
CIDR	O
in	O
1993	O
to	O
replace	O
the	O
previous	O
classful	B-Protocol
network	I-Protocol
addressing	O
architecture	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Its	O
goal	O
was	O
to	O
slow	O
the	O
growth	O
of	O
routing	B-Protocol
tables	I-Protocol
on	O
routers	B-Protocol
across	O
the	O
Internet	O
,	O
and	O
to	O
help	O
slow	O
the	O
rapid	O
exhaustion	B-Protocol
of	I-Protocol
IPv4	I-Protocol
addresses	I-Protocol
.	O
</s>
<s>
IP	B-Protocol
addresses	I-Protocol
are	O
described	O
as	O
consisting	O
of	O
two	O
groups	O
of	O
bits	O
in	O
the	O
address	O
:	O
the	O
most	O
significant	O
bits	O
are	O
the	O
network	O
prefix	O
,	O
which	O
identifies	O
a	O
whole	O
network	O
or	O
subnet	B-Protocol
,	O
and	O
the	O
least	O
significant	O
set	O
forms	O
the	O
host	B-Device
identifier	O
,	O
which	O
specifies	O
a	O
particular	O
interface	O
of	O
a	O
host	B-Device
on	O
that	O
network	O
.	O
</s>
<s>
This	O
division	O
is	O
used	O
as	O
the	O
basis	O
of	O
traffic	O
routing	O
between	O
IP	B-Protocol
networks	I-Protocol
and	O
for	O
address	O
allocation	O
policies	O
.	O
</s>
<s>
Whereas	O
classful	B-Protocol
network	I-Protocol
design	O
for	O
IPv4	B-Protocol
sized	O
the	O
network	O
prefix	O
as	O
one	O
or	O
more	O
8-bit	O
groups	O
,	O
resulting	O
in	O
the	O
blocks	O
of	O
Class	O
A	O
,	O
B	O
,	O
or	O
C	O
addresses	O
,	O
under	O
CIDR	O
address	O
space	O
is	O
allocated	O
to	O
Internet	O
service	O
providers	O
and	O
end	O
users	O
on	O
any	O
address-bit	O
boundary	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
however	O
,	O
the	O
interface	O
identifier	O
has	O
a	O
fixed	O
size	O
of	O
64	O
bits	O
by	O
convention	O
,	O
and	O
smaller	O
subnets	B-Protocol
are	O
never	O
allocated	O
to	O
end	O
users	O
.	O
</s>
<s>
CIDR	O
is	O
based	O
on	O
variable-length	O
subnet	B-Protocol
masking	O
(	O
VLSM	O
)	O
,	O
in	O
which	O
network	O
prefixes	O
have	O
variable	O
length	O
(	O
as	O
opposed	O
to	O
the	O
fixed-length	O
prefixing	O
of	O
the	O
previous	O
classful	B-Protocol
network	I-Protocol
design	O
)	O
.	O
</s>
<s>
The	O
main	O
benefit	O
of	O
this	O
is	O
that	O
it	O
grants	O
finer	O
control	O
of	O
the	O
sizes	O
of	O
subnets	B-Protocol
allocated	O
to	O
organizations	O
,	O
hence	O
slowing	O
the	O
exhaustion	B-Protocol
of	I-Protocol
IPv4	I-Protocol
addresses	I-Protocol
from	O
allocating	O
larger	O
subnets	B-Protocol
than	O
needed	O
.	O
</s>
<s>
CIDR	O
gave	O
rise	O
to	O
a	O
new	O
way	O
of	O
writing	O
IP	B-Protocol
addresses	I-Protocol
known	O
as	O
CIDR	B-Protocol
notation	I-Protocol
,	O
in	O
which	O
an	O
IP	B-Protocol
address	I-Protocol
is	O
followed	O
by	O
a	O
suffix	O
indicating	O
the	O
number	O
of	O
bits	O
of	O
the	O
prefix	O
.	O
</s>
<s>
Some	O
examples	O
of	O
CIDR	B-Protocol
notation	I-Protocol
are	O
the	O
addresses	O
for	O
IPv4	B-Protocol
and	O
for	O
IPv6	B-Protocol
.	O
</s>
<s>
Blocks	O
of	O
addresses	O
having	O
contiguous	O
prefixes	O
may	O
be	O
aggregated	O
as	O
supernets	B-Protocol
,	O
reducing	O
the	O
number	O
of	O
entries	O
in	O
the	O
global	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Each	O
IP	B-Protocol
address	I-Protocol
consists	O
of	O
a	O
network-identifying	O
prefix	O
followed	O
by	O
a	O
host	B-Device
identifier	O
.	O
</s>
<s>
The	O
question	O
is	O
how	O
many	O
bits	O
of	O
the	O
address	O
are	O
in	O
the	O
network	O
prefix	O
,	O
and	O
how	O
many	O
are	O
in	O
the	O
host	B-Device
identifier	O
.	O
</s>
<s>
In	O
the	O
previous	O
IPv4	B-Protocol
classful	B-Protocol
network	I-Protocol
architecture	O
,	O
the	O
top	O
three	O
bits	O
of	O
the	O
32-bit	O
IP	B-Protocol
address	I-Protocol
defined	O
how	O
many	O
bits	O
were	O
in	O
the	O
network	O
prefix	O
:	O
</s>
<s>
The	O
advantage	O
of	O
this	O
system	O
is	O
that	O
the	O
network	O
prefix	O
could	O
be	O
determined	O
for	O
any	O
IP	B-Protocol
address	I-Protocol
without	O
any	O
further	O
information	O
.	O
</s>
<s>
This	O
led	O
to	O
inefficiencies	O
in	O
address	O
use	O
as	O
well	O
as	O
inefficiencies	O
in	O
routing	O
,	O
because	O
it	O
required	O
a	O
large	O
number	O
of	O
allocated	O
class-C	O
networks	O
with	O
individual	O
route	O
announcements	O
,	O
being	O
geographically	O
dispersed	O
with	O
little	O
opportunity	O
for	O
route	B-Protocol
aggregation	I-Protocol
.	O
</s>
<s>
Within	O
a	O
decade	O
after	O
the	O
invention	O
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
,	O
it	O
became	O
apparent	O
that	O
the	O
old	O
classful	B-Protocol
network	I-Protocol
scheme	O
was	O
not	O
scalable	B-Architecture
.	O
</s>
<s>
This	O
led	O
to	O
the	O
development	O
of	O
subnetting	B-Protocol
and	O
CIDR	O
.	O
</s>
<s>
The	O
formerly	O
meaningful	O
class	O
distinctions	O
based	O
on	O
the	O
top	O
3	O
address	O
bits	O
were	O
removed	O
,	O
and	O
the	O
new	O
system	O
was	O
described	O
as	O
being	O
classless	O
,	O
in	O
contrast	O
to	O
the	O
old	O
system	O
,	O
which	O
became	O
known	O
as	O
classful	B-Protocol
.	O
</s>
<s>
Routing	O
protocols	O
were	O
revised	O
to	O
carry	O
not	O
just	O
IP	B-Protocol
addresses	I-Protocol
,	O
but	O
also	O
their	O
matching	O
subnet	B-Protocol
masks	I-Protocol
.	O
</s>
<s>
Implementing	O
CIDR	O
required	O
every	O
host	B-Device
and	O
router	B-Protocol
on	O
the	O
Internet	O
to	O
be	O
reprogrammed	O
in	O
small	O
ways	O
—	O
no	O
small	O
feat	O
at	O
a	O
time	O
when	O
the	O
Internet	O
was	O
entering	O
a	O
period	O
of	O
rapid	O
growth	O
.	O
</s>
<s>
In	O
1993	O
,	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
published	O
a	O
new	O
set	O
of	O
standards	O
,	O
and	O
,	O
to	O
define	O
this	O
new	O
scheme	O
for	O
allocating	O
of	O
IP	B-Protocol
address	I-Protocol
blocks	O
and	O
routing	O
IPv4	B-Protocol
packets	O
.	O
</s>
<s>
After	O
a	O
period	O
of	O
experimentation	O
with	O
various	O
alternatives	O
,	O
Classless	O
Inter-Domain	O
Routing	O
was	O
based	O
on	O
variable-length	O
subnet	B-Protocol
masking	O
(	O
VLSM	O
)	O
,	O
which	O
allows	O
each	O
network	O
to	O
be	O
divided	O
into	O
various	O
power-of-two-sized	O
subnets	B-Protocol
,	O
so	O
that	O
each	O
network	O
or	O
subnet	B-Protocol
can	O
be	O
sized	O
appropriately	O
for	O
local	O
needs	O
.	O
</s>
<s>
Variable-length	O
subnet	B-Protocol
masks	I-Protocol
were	O
mentioned	O
as	O
one	O
alternative	O
in	O
.	O
</s>
<s>
CIDR	B-Protocol
notation	I-Protocol
is	O
a	O
compact	O
representation	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
and	O
its	O
associated	O
network	O
mask	O
.	O
</s>
<s>
CIDR	B-Protocol
notation	I-Protocol
specifies	O
an	O
IP	B-Protocol
address	I-Protocol
,	O
a	O
slash	O
('/'	O
)	O
character	O
,	O
and	O
a	O
decimal	O
number	O
.	O
</s>
<s>
The	O
IP	B-Protocol
address	I-Protocol
in	O
CIDR	B-Protocol
notation	I-Protocol
is	O
always	O
represented	O
according	O
to	O
the	O
standards	O
for	O
IPv4	B-Protocol
or	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
address	O
may	O
denote	O
a	O
specific	O
interface	O
address	O
(	O
including	O
a	O
host	B-Device
identifier	O
,	O
such	O
as	O
)	O
,	O
or	O
it	O
may	O
be	O
the	O
beginning	O
address	O
of	O
an	O
entire	O
network	O
(	O
using	O
a	O
host	B-Device
identifier	O
of	O
0	O
,	O
as	O
in	O
or	O
its	O
equivalent	O
)	O
.	O
</s>
<s>
CIDR	B-Protocol
notation	I-Protocol
can	O
even	O
be	O
used	O
with	O
no	O
IP	B-Protocol
address	I-Protocol
at	O
all	O
,	O
e.g.	O
</s>
<s>
when	O
referring	O
to	O
a	O
as	O
a	O
generic	O
description	O
of	O
an	O
IPv4	B-Protocol
network	I-Protocol
that	O
has	O
a	O
24-bit	O
prefix	O
and	O
8-bit	O
host	B-Device
numbers	O
.	O
</s>
<s>
represents	O
the	O
IPv4	B-Protocol
address	O
and	O
its	O
associated	O
network	O
prefix	O
,	O
or	O
equivalently	O
,	O
its	O
subnet	B-Protocol
mask	I-Protocol
,	O
which	O
has	O
24	O
leading	O
1-bits	O
.	O
</s>
<s>
the	O
IPv4	B-Protocol
block	O
represents	O
the	O
1024	O
IPv4	B-Protocol
addresses	O
from	O
to	O
.	O
</s>
<s>
the	O
IPv6	B-Protocol
block	O
represents	O
the	O
block	O
of	O
IPv6	B-Protocol
addresses	I-Protocol
from	O
to	O
.	O
</s>
<s>
represents	O
the	O
IPv6	B-Protocol
loopback	B-Protocol
address	O
.	O
</s>
<s>
In	O
IPv4	B-Protocol
,	O
what	O
is	O
now	O
called	O
CIDR	B-Protocol
notation	I-Protocol
came	O
into	O
wide	O
use	O
only	O
after	O
the	O
implementation	O
of	O
CIDR	O
.	O
</s>
<s>
It	O
does	O
not	O
appear	O
in	O
the	O
original	O
CIDR	O
standards	O
,	O
which	O
instead	O
used	O
a	O
dotted-decimal	B-Protocol
subnet	B-Protocol
mask	I-Protocol
after	O
the	O
slash	O
;	O
for	O
example	O
,	O
.	O
</s>
<s>
The	O
number	O
of	O
addresses	O
inside	O
a	O
network	O
or	O
subnet	B-Protocol
may	O
be	O
calculated	O
as	O
2address	O
length	O
−	O
prefix	O
length	O
,	O
where	O
address	O
length	O
is	O
128	O
for	O
IPv6	B-Protocol
and	O
32	O
for	O
IPv4	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
in	O
IPv4	B-Protocol
,	O
the	O
prefix	O
length	O
gives	O
:	O
232−29	O
=	O
23	O
=	O
8	O
addresses	O
.	O
</s>
<s>
A	O
subnet	B-Protocol
mask	I-Protocol
is	O
a	O
bitmask	O
that	O
encodes	O
the	O
prefix	O
length	O
associated	O
with	O
an	O
IPv4	B-Protocol
address	O
or	O
network	O
in	O
quad-dotted	B-Protocol
notation	I-Protocol
:	O
32	O
bits	O
,	O
starting	O
with	O
a	O
number	O
of	O
1-bits	O
equal	O
to	O
the	O
prefix	O
length	O
,	O
ending	O
with	O
0-bits	O
,	O
and	O
encoded	O
in	O
four-part	O
dotted-decimal	B-Protocol
format	O
:	O
.	O
</s>
<s>
A	O
subnet	B-Protocol
mask	I-Protocol
encodes	O
the	O
same	O
information	O
as	O
a	O
prefix	O
length	O
but	O
predates	O
the	O
advent	O
of	O
CIDR	O
.	O
</s>
<s>
In	O
CIDR	B-Protocol
notation	I-Protocol
,	O
the	O
prefix	O
bits	O
are	O
always	O
contiguous	O
.	O
</s>
<s>
Subnet	B-Protocol
masks	I-Protocol
were	O
allowed	O
by	O
to	O
specify	O
non-contiguous	O
bits	O
until	O
stated	O
that	O
the	O
mask	O
must	O
be	O
left	O
contiguous	O
.	O
</s>
<s>
Given	O
this	O
constraint	O
,	O
a	O
subnet	B-Protocol
mask	I-Protocol
and	O
CIDR	B-Protocol
notation	I-Protocol
serve	O
exactly	O
the	O
same	O
function	O
.	O
</s>
<s>
CIDR	O
is	O
principally	O
a	O
bitwise	O
,	O
prefix-based	O
standard	O
for	O
the	O
representation	O
of	O
IP	B-Protocol
addresses	I-Protocol
and	O
their	O
routing	O
properties	O
.	O
</s>
<s>
It	O
facilitates	O
routing	O
by	O
allowing	O
blocks	O
of	O
addresses	O
to	O
be	O
grouped	O
into	O
single	O
routing	B-Protocol
table	I-Protocol
entries	O
.	O
</s>
<s>
These	O
groups	O
,	O
commonly	O
called	O
CIDR	O
blocks	O
,	O
share	O
an	O
initial	O
sequence	O
of	O
bits	O
in	O
the	O
binary	O
representation	O
of	O
their	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
IPv4	B-Protocol
CIDR	O
blocks	O
are	O
identified	O
using	O
a	O
syntax	O
similar	O
to	O
that	O
of	O
IPv4	B-Protocol
addresses	O
:	O
a	O
dotted-decimal	B-Protocol
address	O
,	O
followed	O
by	O
a	O
slash	O
,	O
then	O
a	O
number	O
from	O
0	O
to	O
32	O
,	O
i.e.	O
,	O
.	O
</s>
<s>
The	O
dotted	B-Protocol
decimal	I-Protocol
portion	O
is	O
the	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
An	O
IP	B-Protocol
address	I-Protocol
is	O
part	O
of	O
a	O
CIDR	O
block	O
and	O
is	O
said	O
to	O
match	O
the	O
CIDR	O
prefix	O
if	O
the	O
initial	O
n	O
bits	O
of	O
the	O
address	O
and	O
the	O
CIDR	O
prefix	O
are	O
the	O
same	O
.	O
</s>
<s>
An	O
IPv4	B-Protocol
address	O
is	O
32	O
bits	O
so	O
an	O
n-bit	O
CIDR	O
prefix	O
leaves	O
32	O
−	O
n	O
bits	O
unmatched	O
,	O
meaning	O
that	O
232−n	O
IPv4	B-Protocol
addresses	O
match	O
a	O
given	O
n-bit	O
CIDR	O
prefix	O
.	O
</s>
<s>
CIDR	O
is	O
also	O
used	O
for	O
IPv6	B-Protocol
addresses	I-Protocol
and	O
the	O
syntax	O
semantic	O
is	O
identical	O
.	O
</s>
<s>
However	O
,	O
by	O
convention	O
,	O
a	O
subnet	B-Protocol
on	O
broadcast	O
MAC	O
layer	O
networks	O
always	O
has	O
64-bit	O
host	B-Device
identifiers	O
.	O
</s>
<s>
Larger	O
prefixes	O
(	O
/127	O
)	O
are	O
only	O
used	O
on	O
some	O
point-to-point	O
links	O
between	O
routers	B-Protocol
,	O
for	O
security	O
and	O
policy	O
reasons	O
.	O
</s>
<s>
The	O
RIRs	O
,	O
each	O
responsible	O
for	O
a	O
single	O
,	O
large	O
,	O
geographic	O
area	O
,	O
such	O
as	O
Europe	O
or	O
North	O
America	O
,	O
subdivide	O
these	O
blocks	O
and	O
allocate	O
subnets	B-Protocol
to	O
local	O
Internet	O
registries	O
(	O
LIRs	O
)	O
.	O
</s>
<s>
End-user	O
networks	O
receive	O
subnets	B-Protocol
sized	O
according	O
to	O
their	O
projected	O
short-term	O
need	O
.	O
</s>
<s>
Networks	O
served	O
by	O
a	O
single	O
ISP	O
are	O
encouraged	O
by	O
IETF	O
recommendations	O
to	O
obtain	O
IP	B-Protocol
address	I-Protocol
space	O
directly	O
from	O
their	O
ISP	O
.	O
</s>
<s>
Networks	O
served	O
by	O
multiple	O
ISPs	O
,	O
on	O
the	O
other	O
hand	O
,	O
may	O
obtain	O
provider-independent	B-Protocol
address	I-Protocol
space	I-Protocol
directly	O
from	O
the	O
appropriate	O
RIR	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
late	O
1990s	O
,	O
the	O
IP	B-Protocol
address	I-Protocol
(	O
since	O
reassigned	O
)	O
was	O
used	O
by	O
www.freesoft.org	O
.	O
</s>
<s>
,	O
a	O
large	O
CIDR	O
block	O
containing	O
over	O
2	O
million	O
addresses	O
,	O
had	O
been	O
assigned	O
by	O
ARIN	O
(	O
the	O
North	O
American	O
RIR	O
)	O
to	O
MCI	B-Application
.	O
</s>
<s>
Automation	O
Research	O
Systems	O
(	O
ARS	O
)	O
,	O
a	O
Virginia	O
VAR	O
,	O
leased	O
an	O
Internet	O
connection	O
from	O
MCI	B-Application
and	O
was	O
assigned	O
the	O
block	O
,	O
capable	O
of	O
addressing	O
just	O
over	O
1000	O
devices	O
.	O
</s>
<s>
Outside	O
MCI	B-Application
's	O
network	O
,	O
the	O
prefix	O
would	O
be	O
used	O
to	O
direct	O
to	O
MCI	B-Application
traffic	O
bound	O
not	O
only	O
for	O
,	O
but	O
also	O
for	O
any	O
of	O
the	O
roughly	O
two	O
million	O
IP	B-Protocol
addresses	I-Protocol
with	O
the	O
same	O
initial	O
11	O
bits	O
.	O
</s>
<s>
Within	O
MCI	B-Application
's	O
network	O
,	O
would	O
become	O
visible	O
,	O
directing	O
traffic	O
to	O
the	O
leased	O
line	O
serving	O
ARS	O
.	O
</s>
<s>
Addressformat	O
Differenceto	O
last	O
address	B-Protocol
Mask	I-Protocol
Addresses	O
Relativeto	O
classA	O
,	O
B	O
,	O
C	O
Restrictionson	O
a	O
,	O
b	O
,	O
c	O
and	O
d( 	O
0	O
..	O
255	O
unless	O
noted	O
)	O
Typical	O
use	O
Decimal2n	O
+120	O
C	O
Host	B-Device
route	O
+221	O
Cd	O
=	O
0	O
...	O
(	O
2n	O
)	O
...	O
254	O
Point-to-point	O
links	O
(	O
)	O
+422	O
Cd	O
=	O
0	O
...	O
(	O
4n	O
)	O
...	O
252	O
+823	O
Cd	O
=	O
0	O
...	O
(	O
8n	O
)	O
...	O
248	O
Smallest	O
multi-host	O
network	O
+1624	O
Cd	O
=	O
0	O
...	O
(	O
16n	O
)	O
...	O
240	O
Small	O
LAN	B-General_Concept
+3225	O
Cd	O
=	O
0	O
...	O
(	O
32n	O
)	O
...	O
224	O
+6426	O
Cd	O
=	O
0	O
,	O
64	O
,	O
128	O
,	O
192	O
+12827	O
Cd	O
=	O
0	O
,	O
128	O
Large	O
LAN	B-General_Concept
+256281	O
C	O
+512292	O
Cc	O
=	O
0	O
...	O
(	O
2n	O
)	O
...	O
254	O
+1	O
,	O
0242104	O
Cc	O
=	O
0	O
...	O
(	O
4n	O
)	O
...	O
252Small	O
business	O
+2	O
,	O
0482118	O
Cc	O
=	O
0	O
...	O
(	O
8n	O
)	O
...	O
248	O
Small	O
ISP/	O
large	O
business	O
+4	O
,	O
09621216	O
Cc	O
=	O
0	O
...	O
(	O
16n	O
)	O
...	O
240	O
+8	O
,	O
19221332	O
Cc	O
=	O
0	O
...	O
(	O
32n	O
)	O
...	O
224	O
ISP/	O
large	O
business	O
+16	O
,	O
38421464	O
Cc	O
=	O
0	O
,	O
64	O
,	O
128	O
,	O
192	O
+32	O
,	O
768215128	O
Cc	O
=	O
0	O
,	O
128	O
+65	O
,	O
536216256	O
C	O
=	O
B	O
+131	O
,	O
0722172	O
Bb	O
=	O
0	O
...	O
(	O
2n	O
)	O
...	O
254	O
+262	O
,	O
1442184	O
Bb	O
=	O
0	O
...	O
(	O
4n	O
)	O
...	O
252	O
+524	O
,	O
2882198	O
Bb	O
=	O
0	O
...	O
(	O
8n	O
)	O
...	O
248	O
+1	O
,	O
048	O
,	O
57622016	O
Bb	O
=	O
0	O
...	O
(	O
16n	O
)	O
...	O
240	O
+2	O
,	O
097	O
,	O
15222132	O
Bb	O
=	O
0	O
...	O
(	O
32n	O
)	O
...	O
224	O
+4	O
,	O
194	O
,	O
30422264	O
Bb	O
=	O
0	O
,	O
64	O
,	O
128	O
,	O
192	O
+8	O
,	O
388	O
,	O
608223128	O
Bb	O
=	O
0	O
,	O
128	O
+16	O
,	O
777	O
,	O
216224256	O
B	O
=	O
A	O
Largest	O
IANA	O
block	O
allocation	O
+33	O
,	O
554	O
,	O
4322252	O
Aa	O
=	O
0	O
...	O
(	O
2n	O
)	O
...	O
254	O
+67	O
,	O
108	O
,	O
8642264	O
Aa	O
=	O
0	O
...	O
(	O
4n	O
)	O
...	O
252	O
+134	O
,	O
217	O
,	O
7282278	O
Aa	O
=	O
0	O
...	O
(	O
8n	O
)	O
...	O
248	O
+268	O
,	O
435	O
,	O
45622816	O
Aa	O
=	O
0	O
...	O
(	O
16n	O
)	O
...	O
240	O
+536	O
,	O
870	O
,	O
91222932	O
Aa	O
=	O
0	O
...	O
(	O
32n	O
)	O
...	O
224	O
+1	O
,	O
073	O
,	O
741	O
,	O
82423064	O
Aa	O
=	O
0	O
,	O
64	O
,	O
128	O
,	O
192	O
+2	O
,	O
147	O
,	O
483	O
,	O
648231128	O
Aa	O
=	O
0	O
,	O
128	O
+4	O
,	O
294	O
,	O
967	O
,	O
296232256	O
A	O
Entire	O
IPv4	B-Protocol
Internet	I-Protocol
,	O
default	B-Protocol
route	I-Protocol
.	O
</s>
<s>
In	O
common	O
usage	O
,	O
the	O
first	O
address	O
in	O
a	O
subnet	B-Protocol
,	O
all	O
binary	O
zero	O
in	O
the	O
host	B-Device
identifier	O
,	O
is	O
reserved	O
for	O
referring	O
to	O
the	O
network	O
itself	O
,	O
while	O
the	O
last	O
address	O
,	O
all	O
binary	O
one	O
in	O
the	O
host	B-Device
identifier	O
,	O
is	O
used	O
as	O
a	O
broadcast	B-Protocol
address	I-Protocol
for	O
the	O
network	O
;	O
this	O
reduces	O
the	O
number	O
of	O
addresses	O
available	O
for	O
hosts	O
by	O
2	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
a	O
network	O
,	O
with	O
one	O
binary	O
digit	O
in	O
the	O
host	B-Device
identifier	O
,	O
would	O
be	O
unusable	O
,	O
as	O
such	O
a	O
subnet	B-Protocol
would	O
provide	O
no	O
available	O
host	B-Device
addresses	O
after	O
this	O
reduction	O
.	O
</s>
<s>
creates	O
an	O
exception	O
to	O
the	O
"	O
host	B-Device
all	O
ones	O
"	O
and	O
"	O
host	B-Device
all	O
zeros	O
"	O
rules	O
to	O
make	O
networks	O
usable	O
for	O
point-to-point	O
links	O
.	O
</s>
<s>
addresses	O
(	O
single-host	O
network	O
)	O
must	O
be	O
accessed	O
by	O
explicit	O
routing	O
rules	O
,	O
as	O
there	O
is	O
no	O
room	O
in	O
such	O
a	O
network	O
for	O
a	O
gateway	O
.	O
</s>
<s>
In	O
routed	O
subnets	B-Protocol
larger	O
than	O
or	O
,	O
the	O
number	O
of	O
available	O
host	B-Device
addresses	O
is	O
usually	O
reduced	O
by	O
two	O
,	O
namely	O
the	O
largest	O
address	O
,	O
which	O
is	O
reserved	O
as	O
the	O
broadcast	B-Protocol
address	I-Protocol
,	O
and	O
the	O
smallest	O
address	O
,	O
which	O
identifies	O
the	O
network	O
itself	O
.	O
</s>
<s>
The	O
large	O
address	O
size	O
used	O
in	O
IPv6	B-Protocol
permitted	O
implementation	O
of	O
worldwide	O
route	B-Protocol
summarization	I-Protocol
and	O
guaranteed	O
sufficient	O
address	O
pools	O
at	O
each	O
site	O
.	O
</s>
<s>
The	O
standard	O
subnet	B-Protocol
size	O
for	O
IPv6	B-Protocol
networks	O
is	O
a	O
block	O
,	O
which	O
is	O
required	O
for	O
the	O
operation	O
of	O
stateless	O
address	O
autoconfiguration	O
.	O
</s>
<s>
This	O
IPv6	B-Protocol
subnetting	B-Protocol
reference	O
lists	O
the	O
sizes	O
for	O
IPv6	B-Protocol
subnetworks	B-Protocol
.	O
</s>
<s>
Different	O
types	O
of	O
network	O
links	O
may	O
require	O
different	O
subnet	B-Protocol
sizes	O
.	O
</s>
<s>
The	O
subnet	B-Protocol
mask	I-Protocol
separates	O
the	O
bits	O
of	O
the	O
network	O
identifier	O
prefix	O
from	O
the	O
bits	O
of	O
the	O
interface	O
identifier	O
.	O
</s>
<s>
CIDR	O
provides	O
fine-grained	O
routing	B-Protocol
prefix	I-Protocol
aggregation	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
first	O
20	O
bits	O
of	O
their	O
network	O
prefixes	O
match	O
,	O
sixteen	O
contiguous	O
networks	O
can	O
be	O
aggregated	O
and	O
advertised	O
to	O
a	O
larger	O
network	O
as	O
a	O
single	O
routing	B-Protocol
table	I-Protocol
entry	O
.	O
</s>
