<s>
A	O
classful	B-Protocol
network	I-Protocol
is	O
an	O
obsolete	O
network	O
addressing	O
architecture	O
used	O
in	O
the	O
Internet	O
from	O
1981	O
until	O
the	O
introduction	O
of	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
in	O
1993	O
.	O
</s>
<s>
The	O
method	O
divides	O
the	O
IP	B-Protocol
address	I-Protocol
space	O
for	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
into	O
five	O
address	O
classes	O
based	O
on	O
the	O
leading	O
four	O
address	O
bits	O
.	O
</s>
<s>
Classes	O
A	O
,	O
B	O
,	O
and	O
C	O
provide	O
unicast	B-Protocol
addresses	O
for	O
networks	O
of	O
three	O
different	O
network	O
sizes	O
.	O
</s>
<s>
Since	O
its	O
discontinuation	O
,	O
remnants	O
of	O
classful	B-Protocol
network	I-Protocol
concepts	O
have	O
remained	O
in	O
practice	O
only	O
in	O
limited	O
scope	O
in	O
the	O
default	O
configuration	O
parameters	O
of	O
some	O
network	O
software	O
and	O
hardware	O
components	O
,	O
most	O
notably	O
in	O
the	O
default	O
configuration	O
of	O
subnet	B-Protocol
masks	I-Protocol
.	O
</s>
<s>
In	O
the	O
original	O
address	O
definition	O
,	O
the	O
most	O
significant	O
eight	O
bits	O
of	O
the	O
32-bit	O
IPv4	B-Protocol
address	O
was	O
the	O
network	O
number	O
field	O
which	O
specified	O
the	O
particular	O
network	O
a	O
host	O
was	O
attached	O
to	O
.	O
</s>
<s>
This	O
format	O
was	O
sufficient	O
at	O
a	O
time	O
when	O
only	O
a	O
few	O
large	O
networks	O
existed	O
,	O
such	O
as	O
the	O
ARPANET	O
(	O
network	O
number	O
10	O
)	O
,	O
and	O
before	O
the	O
wide	O
proliferation	O
of	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
(	O
LANs	B-General_Concept
)	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
some	O
organizations	O
involved	O
in	O
the	O
early	O
development	O
of	O
the	O
Internet	O
received	O
address	O
space	O
allocations	O
far	O
larger	O
than	O
they	O
would	O
ever	O
need	O
(	O
16,777,216	O
IP	B-Protocol
addresses	I-Protocol
each	O
)	O
.	O
</s>
<s>
It	O
became	O
clear	O
early	O
in	O
the	O
growth	O
of	O
the	O
network	O
that	O
this	O
would	O
be	O
a	O
critical	O
scalability	B-Architecture
limitation	O
.	O
</s>
<s>
Expansion	O
of	O
the	O
network	O
had	O
to	O
ensure	O
compatibility	O
with	O
the	O
existing	O
address	O
space	O
and	O
the	O
IPv4	B-Protocol
packet	O
structure	O
,	O
and	O
avoid	O
the	O
renumbering	O
of	O
the	O
existing	O
networks	O
.	O
</s>
<s>
A	O
Class	B-Protocol
B	I-Protocol
network	I-Protocol
was	O
a	O
network	O
in	O
which	O
all	O
addresses	O
had	O
the	O
two	O
most-significant	O
bits	O
set	O
to	O
1	O
and	O
0	O
respectively	O
.	O
</s>
<s>
For	O
these	O
networks	O
,	O
the	O
network	B-Protocol
address	I-Protocol
was	O
given	O
by	O
the	O
next	O
14	O
bits	O
of	O
the	O
address	O
,	O
thus	O
leaving	O
16	O
bits	O
for	O
numbering	O
host	O
on	O
the	O
network	O
for	O
a	O
total	O
of	O
addresses	O
per	O
network	O
.	O
</s>
<s>
This	O
architecture	O
change	O
extended	O
the	O
addressing	O
capacity	O
of	O
the	O
Internet	O
but	O
did	O
not	O
prevent	O
IP	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
.	O
</s>
<s>
The	O
problem	O
was	O
that	O
many	O
sites	O
needed	O
larger	O
address	O
blocks	O
than	O
a	O
Class	B-Protocol
C	I-Protocol
network	I-Protocol
provided	O
,	O
and	O
therefore	O
they	O
received	O
a	O
Class	O
B	O
block	O
,	O
which	O
was	O
in	O
most	O
cases	O
much	O
larger	O
than	O
required	O
.	O
</s>
<s>
Starting	O
in	O
1993	O
,	O
classful	B-Protocol
networking	O
was	O
replaced	O
by	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
,	O
in	O
an	O
attempt	O
to	O
solve	O
this	O
problem	O
.	O
</s>
<s>
Under	O
classful	B-Protocol
network	I-Protocol
addressing	O
,	O
the	O
32-bit	O
IPv4	B-Protocol
address	O
space	O
was	O
partitioned	O
into	O
5	O
classes	O
(	O
A-E	O
)	O
as	O
shown	O
in	O
the	O
following	O
tables	O
.	O
</s>
<s>
Class	O
Leading	O
bits	O
Size	O
of	O
network	O
number	O
bit	O
field	O
Size	O
of	O
rest	O
bit	O
field	O
Number	O
of	O
networks	O
Addresses	O
per	O
network	O
Total	O
addresses	O
in	O
class	O
Start	O
address	O
End	O
address	O
Default	O
subnet	B-Protocol
mask	I-Protocol
in	O
dot-decimal	B-Protocol
notation	I-Protocol
CIDR	B-Protocol
notation	I-Protocol
Class	O
A	O
0	O
8	O
24	O
128	O
(	O
27	O
)	O
16,777,216	O
(	O
224	O
)	O
2,147,483,648	O
(	O
231	O
)	O
Class	O
B	O
10	O
16	O
16	O
16,384	O
(	O
214	O
)	O
65,536	O
(	O
216	O
)	O
1,073,741,824	O
(	O
230	O
)	O
Class	O
C	O
110	O
24	O
8	O
2,097,152	O
(	O
221	O
)	O
256	O
(	O
28	O
)	O
536,870,912	O
(	O
229	O
)	O
Class	O
D	O
(	O
multicast	O
)	O
1110	O
not	O
defined	O
not	O
defined	O
not	O
defined	O
not	O
defined	O
268,435,456	O
(	O
228	O
)	O
not	O
defined	O
/4MULTICAST_IP_ADDR	O
.	O
</s>
<s>
In	O
network	O
prefix	O
or	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
notation	O
,	O
IP	O
multicast	O
addresses	O
are	O
summarized	O
as	O
224.0.0.0/4	O
.	O
</s>
<s>
The	O
number	O
of	O
addresses	O
usable	O
for	O
addressing	O
specific	O
hosts	O
in	O
each	O
network	O
is	O
always	O
,	O
where	O
N	O
is	O
the	O
number	O
of	O
rest	O
field	O
bits	O
,	O
and	O
the	O
subtraction	O
of	O
2	O
adjusts	O
for	O
the	O
use	O
of	O
the	O
all-bits-zero	O
host	O
value	O
to	O
represent	O
the	O
network	B-Protocol
address	I-Protocol
and	O
the	O
all-bits-one	O
host	O
value	O
for	O
use	O
as	O
a	O
broadcast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Today	O
,	O
IP	B-Protocol
addresses	I-Protocol
are	O
associated	O
with	O
a	O
subnet	B-Protocol
mask	I-Protocol
.	O
</s>
<s>
This	O
was	O
not	O
required	O
in	O
a	O
classful	B-Protocol
network	I-Protocol
because	O
the	O
mask	O
was	O
implied	O
by	O
the	O
address	O
itself	O
;	O
any	O
network	O
device	O
would	O
inspect	O
the	O
first	O
few	O
bits	O
of	O
the	O
IP	B-Protocol
address	I-Protocol
to	O
determine	O
the	O
class	O
of	O
the	O
address	O
and	O
thus	O
its	O
netmask	B-Protocol
.	O
</s>
<s>
The	O
blocks	O
numerically	O
at	O
the	O
start	O
and	O
end	O
of	O
classes	O
A	O
,	O
B	O
and	O
C	O
were	O
originally	O
reserved	O
for	O
special	O
addressing	O
or	O
future	O
features	O
,	O
i.e.	O
,	O
and	O
are	O
reserved	O
in	O
former	O
class	O
A	O
;	O
and	O
were	O
reserved	O
in	O
former	O
class	O
B	O
but	O
are	O
now	O
available	O
for	O
assignment	O
;	O
and	O
are	O
reserved	O
in	O
former	O
class	O
C	O
.	O
While	O
the	O
network	O
is	O
a	O
Class	O
A	O
network	O
,	O
it	O
is	O
designated	O
for	O
loopback	B-Protocol
and	O
cannot	O
be	O
assigned	O
to	O
a	O
network	O
.	O
</s>
<s>
Class	O
D	O
is	O
reserved	O
for	O
multicast	O
and	O
cannot	O
be	O
used	O
for	O
regular	O
unicast	B-Protocol
traffic	O
.	O
</s>
<s>
Many	O
older	O
routers	B-Protocol
will	O
not	O
accept	O
using	O
it	O
in	O
any	O
context	O
.	O
</s>
