Editing Classful network (section)

==Classful addressing definition==
Under classful network addressing, the 32-bit IPv4 address space was partitioned into five classes (A-E) as shown in the following tables.
;Classes
{| class="wikitable"
! Class
! Leading bits
! Size of ''network number'' bit field
! Size of ''rest'' bit field
! Number of networks
! Addresses per network
! Total addresses in class
! Start address
! End address
! Default [[subnet mask]] in [[dot-decimal notation]]
! [[CIDR notation]]
|-
| Class A
| 0
| 8
| 24
| 128 (2<sup>7</sup>)
| 16,777,216 (2<sup>24</sup>)
| 2,147,483,648 (2<sup>31</sup>)
| {{IPaddr|0.0.0.0}}
| {{IPaddr|127.255.255.255}}{{efn|{{IPaddr|127.0.0.0}} through {{IPaddr|127.255.255.255}} are reserved for [[localhost|loopback addresses]]. Although reserved, they are still part of the class A address group.}}
| {{IPaddr||8|netmask=dotted}}
| {{IPaddr||8}}
|-
| Class B
| 10
| 16
| 16
| 16,384 (2<sup>14</sup>)
| 65,536 (2<sup>16</sup>)
| 1,073,741,824 (2<sup>30</sup>)
| {{IPaddr|128.0.0.0}}
| {{IPaddr|191.255.255.255}}
| {{IPaddr||16|netmask=dotted}}
| {{IPaddr||16}}
|-
| Class C
| 110
| 24
| 8
| 2,097,152 (2<sup>21</sup>)
| 256 (2<sup>8</sup>)
| 536,870,912 (2<sup>29</sup>)
| {{IPaddr|192.0.0.0}}
| {{IPaddr|223.255.255.255}}
| {{IPaddr||24|netmask=dotted}}
| {{IPaddr||24}}
|-
| Class D ([[multicast]])
| 1110
| not defined
| not defined
| not defined
| not defined
| 268,435,456 (2<sup>28</sup>)
| {{IPaddr|224.0.0.0}}
| {{IPaddr|239.255.255.255}}
| not defined
| /4<ref>[https://www.ge.com/digital/documentation/cimplicity/version10/oxy_ex-2/advanced_features/topics/g_cimplicity_advanced_features_multicast_ip_addr.html MULTICAST_IP_ADDR]. [[General Electric]] Digital Solutions. [[CIMPLICITY]] 10.0. <q>In network prefix or Classless Inter-Domain Routing ( CIDR) notation, IP multicast addresses are summarized as 224.0.0.0/4.</q></ref>
|-
| Class E (reserved)
| 1111
| not defined
| not defined
| not defined
| not defined
| 268,435,456 (2<sup>28</sup>)
| {{IPaddr|240.0.0.0}}
| {{IPaddr|255.255.255.255}}{{efn|{{IPaddr|255.255.255.255}} is reserved as the IPv4 [[broadcast address]].}}
| not defined
| not defined
|}

;Bit-wise representation
In the following bit-wise representation,
* ''n'' indicates a bit used for the network ID.
* ''H'' indicates a bit used for the host ID.
* ''X'' indicates a bit without a specified purpose.
<!--127.255.255.255 is correct for Class A-->
<pre>
Class A
  0.  0.  0.  0 = 00000000.00000000.00000000.00000000
127.255.255.255 = 01111111.11111111.11111111.11111111
                  0nnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B
128.  0.  0.  0 = 10000000.00000000.00000000.00000000
191.255.255.255 = 10111111.11111111.11111111.11111111
                  10nnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH

Class C
192.  0.  0.  0 = 11000000.00000000.00000000.00000000
223.255.255.255 = 11011111.11111111.11111111.11111111
                  110nnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH

Class D
224.  0.  0.  0 = 11100000.00000000.00000000.00000000
239.255.255.255 = 11101111.11111111.11111111.11111111
                  1110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

Class E
240.  0.  0.  0 = 11110000.00000000.00000000.00000000
255.255.255.255 = 11111111.11111111.11111111.11111111
                  1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX
</pre>

The number of addresses usable for addressing specific hosts in each network is always {{nowrap|2<sup>N</sup> - 2}}, where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host value to represent the network address and the all-bits-one host value for use as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the maximum number of hosts is 254.

Today, IP addresses are associated with a [[subnet mask]]. This was not required in a classful network because the mask was implied by the address itself; any network device would inspect the first few bits of the IP address to determine the class of the address and thus its netmask.

The blocks numerically at the start and end of classes A, B and C were originally reserved for special addressing or future features, i.e., {{IPaddr|0.0.0.0|8}} and {{IPaddr|127.0.0.0|8}} are reserved in former class A; {{IPaddr|128.0.0.0|16}} and {{IPaddr|191.255.0.0|16}} were reserved in former class B but are now available for assignment; {{IPaddr|192.0.0.0|24}} and {{IPaddr|223.255.255.0|24}} are reserved in former class C. While the {{IPaddr|127.0.0.0|8}} network is a Class A network, it is designated for [[loopback]] and cannot be assigned to a network.<ref name="RFC 5735">{{cite IETF |title=Special Use IPv4 Addresses |author1=M. Cotton |author2=L. Vegoda |date=January 2010 |RFC=5735}}</ref>

Class D is reserved for [[multicast]] and cannot be used for regular unicast traffic. Class E is reserved and cannot be used on the public Internet. Many older routers will not accept using it in any context.{{citation needed|date=July 2018}}