Editing Classless Inter-Domain Routing (section)

===IPv4 CIDR blocks===
{| class="wikitable"
! rowspan = 2 | Address<br />format
! rowspan = 2 | Difference<br />to last address
! rowspan = 2 | Mask
! colspan = 2 | Addresses
! rowspan = 2 | Relative<br />to class<br />A, B, C
! rowspan = 2 | Restrictions<br />on ''a'', ''b'', ''c'' and ''d''<br /><small>(0..255 unless noted)</small>
! rowspan = 2 | Typical use
|-
! Decimal
!2<sup>''n''</sup>
|-
|{{IPaddr|a.b.c.d|32}}
| 
|{{IPaddr|255.255.255.255|}}
| style="text-align:right;" |1
| style="text-align:right;" |2<sup>0</sup>
| style="text-align:right;" |{{frac|1|256}} C
|
| Host route
|-
|{{IPaddr|a.b.c.d|31}}
| +{{IPaddr|0.0.0.1|}}
|{{IPaddr|255.255.255.254|}}
| style="text-align:right;" |2
| style="text-align:right;" |2<sup>1</sup>
| style="text-align:right;" |{{frac|1|128}} C
|''d'' = 0 ... (2''n'') ... 254
| Point-to-point links ({{IETF RFC|3021}})
|-
|{{IPaddr|a.b.c.d|30}}
| +{{IPaddr|0.0.0.3|}}
|{{IPaddr|255.255.255.252|}}
| style="text-align:right;" |4
| style="text-align:right;" |2<sup>2</sup>
| style="text-align:right;" |{{frac|1|64}} C
|''d'' = 0 ... (4''n'') ... 252
| {{nowrap|Point-to-point links (glue network)}}
|-
|{{IPaddr|a.b.c.d|29}}
| +{{IPaddr|0.0.0.7|}}
|{{IPaddr|255.255.255.248|}}
| style="text-align:right;" |8
| style="text-align:right;" |2<sup>3</sup>
| style="text-align:right;" |{{frac|1|32}} C
|''d'' = 0 ... (8''n'') ... 248
| Smallest multi-host network
|-
|{{IPaddr|a.b.c.d|28}}
| +{{IPaddr|0.0.0.15|}}
|{{IPaddr|255.255.255.240|}}
| style="text-align:right;" |16
| style="text-align:right;" |2<sup>4</sup>
| style="text-align:right;" |{{frac|1|16}} C
|''d'' = 0 ... (16''n'') ... 240
| rowspan=3 | Small [[LAN]]
|-
|{{IPaddr|a.b.c.d|27}}
| +{{IPaddr|0.0.0.31|}}
|{{IPaddr|255.255.255.224|}}
| style="text-align:right;" |32
| style="text-align:right;" |2<sup>5</sup>
| style="text-align:right;" |{{frac|1|8}} C
|''d'' = 0 ... (32''n'') ... 224
|-
|{{IPaddr|a.b.c.d|26}}
| +{{IPaddr|0.0.0.63|}}
|{{IPaddr|255.255.255.192|}}
| style="text-align:right;" |64
| style="text-align:right;" |2<sup>6</sup>
| style="text-align:right;" |{{frac|1|4}} C
|''d'' = 0, 64, 128, 192
|-
|{{IPaddr|a.b.c.d|25}}
| +{{IPaddr|0.0.0.127|}}
|{{IPaddr|255.255.255.128|}}
| style="text-align:right;" |128
| style="text-align:right;" |2<sup>7</sup>
| style="text-align:right;" |{{frac|1|2}} C
|''d'' = 0, 128
| rowspan=2 | Large [[LAN]]
|-
|{{IPaddr|a.b.c.0|24}}
| +{{IPaddr|0.0.0.255|}}
|{{IPaddr|255.255.255.0|}}
| style="text-align:right;" |256
| style="text-align:right;" |2<sup>8</sup>
| style="text-align:right;" |1 C
|
|-
|{{IPaddr|a.b.c.0|23}}
| +{{IPaddr|0.0.1.255|}}
|{{IPaddr|255.255.254.0|}}
| style="text-align:right;" |512
| style="text-align:right;" |2<sup>9</sup>
| style="text-align:right;" |2 C
|''c'' = 0 ... (2''n'') ... 254
|
|-
|{{IPaddr|a.b.c.0|22}}
| +{{IPaddr|0.0.3.255|}}
|{{IPaddr|255.255.252.0|}}
| style="text-align:right;" |1,024
| style="text-align:right;" |2<sup>10</sup>
| style="text-align:right;" |4 C
|''c'' = 0 ... (4''n'') ... 252
|Small business
|-
|{{IPaddr|a.b.c.0|21}}
| +{{IPaddr|0.0.7.255|}}
|{{IPaddr|255.255.248.0|}}
| style="text-align:right;" |2,048
| style="text-align:right;" |2<sup>11</sup>
| style="text-align:right;" |8 C
|''c'' = 0 ... (8''n'') ... 248
| rowspan=2 | Small [[ISP]]/ large business
|-
|{{IPaddr|a.b.c.0|20}}
| +{{IPaddr|0.0.15.255|}}
|{{IPaddr|255.255.240.0|}}
| style="text-align:right;" |4,096
| style="text-align:right;" |2<sup>12</sup>
| style="text-align:right;" |16 C
|''c'' = 0 ... (16''n'') ... 240
|-
|{{IPaddr|a.b.c.0|19}}
| +{{IPaddr|0.0.31.255|}}
|{{IPaddr|255.255.224.0|}}
| style="text-align:right;" |8,192
| style="text-align:right;" |2<sup>13</sup>
| style="text-align:right;" |32 C
|''c'' = 0 ... (32''n'') ... 224
| rowspan=3 | [[ISP]]/ large business
|-
|{{IPaddr|a.b.c.0|18}}
| +{{IPaddr|0.0.63.255|}}
|{{IPaddr|255.255.192.0|}}
| style="text-align:right;" |16,384
| style="text-align:right;" |2<sup>14</sup>
| style="text-align:right;" |64 C
|''c'' = 0, 64, 128, 192
|-
|{{IPaddr|a.b.c.0|17}}
| +{{IPaddr|0.0.127.255|}}
|{{IPaddr|255.255.128.0|}}
| style="text-align:right;" |32,768
| style="text-align:right;" |2<sup>15</sup>
| style="text-align:right;" |128 C
|''c'' = 0, 128
|-
|{{IPaddr|a.b.0.0|16}}
| +{{IPaddr|0.0.255.255|}}
|{{IPaddr|255.255.0.0|}}
| style="text-align:right;" |65,536
| style="text-align:right;" |2<sup>16</sup>
| style="text-align:right;" |256 C = B
|
|
|-
|{{IPaddr|a.b.0.0|15}}
| +{{IPaddr|0.1.255.255|}}
|{{IPaddr|255.254.0.0|}}
| style="text-align:right;" |131,072
| style="text-align:right;" |2<sup>17</sup>
| style="text-align:right;" |2 B
|''b'' = 0 ... (2''n'') ... 254
|
|-
|{{IPaddr|a.b.0.0|14}}
| +{{IPaddr|0.3.255.255|}}
|{{IPaddr|255.252.0.0|}}
| style="text-align:right;" |262,144
| style="text-align:right;" |2<sup>18</sup>
| style="text-align:right;" |4 B
|''b'' = 0 ... (4''n'') ... 252
|
|-
|{{IPaddr|a.b.0.0|13}}
| +{{IPaddr|0.7.255.255|}}
|{{IPaddr|255.248.0.0|}}
| style="text-align:right;" |524,288
| style="text-align:right;" |2<sup>19</sup>
| style="text-align:right;" |8 B
|''b'' = 0 ... (8''n'') ... 248
|
|-
|{{IPaddr|a.b.0.0|12}}
| +{{IPaddr|0.15.255.255|}}
|{{IPaddr|255.240.0.0|}}
| style="text-align:right;" |1,048,576
| style="text-align:right;" |2<sup>20</sup>
| style="text-align:right;" |16 B
|''b'' = 0 ... (16''n'') ... 240
|
|-
|{{IPaddr|a.b.0.0|11}}
| +{{IPaddr|0.31.255.255|}}
|{{IPaddr|255.224.0.0|}}
| style="text-align:right;" |2,097,152
| style="text-align:right;" |2<sup>21</sup>
| style="text-align:right;" |32 B
|''b'' = 0 ... (32''n'') ... 224
|
|-
|{{IPaddr|a.b.0.0|10}}
| +{{IPaddr|0.63.255.255|}}
|{{IPaddr|255.192.0.0|}}
| style="text-align:right;" |4,194,304
| style="text-align:right;" |2<sup>22</sup>
| style="text-align:right;" |64 B
|''b'' = 0, 64, 128, 192
|
|-
|{{IPaddr|a.b.0.0|9}}
| +{{IPaddr|0.127.255.255|}}
|{{IPaddr|255.128.0.0|}}
| style="text-align:right;" |8,388,608
| style="text-align:right;" |2<sup>23</sup>
| style="text-align:right;" |128 B
|''b'' = 0, 128
|
|-
|{{IPaddr|a.0.0.0|8}}
| +{{IPaddr|0.255.255.255|}}
|{{IPaddr|255.0.0.0|}}
| style="text-align:right;" |16,777,216
| style="text-align:right;" |2<sup>24</sup>
| style="text-align:right;" |256 B = A
|
| Largest [[IANA]] block allocation
|-
|{{IPaddr|a.0.0.0|7}}
| +{{IPaddr|1.255.255.255|}}
|{{IPaddr|254.0.0.0|}}
| style="text-align:right;" |33,554,432
| style="text-align:right;" |2<sup>25</sup>
| style="text-align:right;" |2 A
|''a'' = 0 ... (2''n'') ... 254
|
|-
|{{IPaddr|a.0.0.0|6}}
| +{{IPaddr|3.255.255.255|}}
|{{IPaddr|252.0.0.0|}}
| style="text-align:right;" |67,108,864
| style="text-align:right;" |2<sup>26</sup>
| style="text-align:right;" |4 A
|''a'' = 0 ... (4''n'') ... 252
|
|-
|{{IPaddr|a.0.0.0|5}}
| +{{IPaddr|7.255.255.255|}}
|{{IPaddr|248.0.0.0|}}
| style="text-align:right;" |134,217,728
| style="text-align:right;" |2<sup>27</sup>
| style="text-align:right;" |8 A
|''a'' = 0 ... (8''n'') ... 248
|
|-
|{{IPaddr|a.0.0.0|4}}
| +{{IPaddr|15.255.255.255|}}
|{{IPaddr|240.0.0.0|}}
| style="text-align:right;" |268,435,456
| style="text-align:right;" |2<sup>28</sup>
| style="text-align:right;" |16 A
|''a'' = 0 ... (16''n'') ... 240
|
|-
|{{IPaddr|a.0.0.0|3}}
| +{{IPaddr|31.255.255.255|}}
|{{IPaddr|224.0.0.0|}}
| style="text-align:right;" |536,870,912
| style="text-align:right;" |2<sup>29</sup>
| style="text-align:right;" |32 A
|''a'' = 0 ... (32''n'') ... 224
|
|-
|{{IPaddr|a.0.0.0|2}}
| +{{IPaddr|63.255.255.255|}}
|{{IPaddr|192.0.0.0|}}
| style="text-align:right;" |1,073,741,824
| style="text-align:right;" |2<sup>30</sup>
| style="text-align:right;" |64 A
|''a'' = 0, 64, 128, 192
|
|-
|{{IPaddr|a.0.0.0|1}}
| +{{IPaddr|127.255.255.255|}}
|{{IPaddr|128.0.0.0|}}
| style="text-align:right;" |2,147,483,648
| style="text-align:right;" |2<sup>31</sup>
| style="text-align:right;" |128 A
|''a'' = 0, 128
|
|-
|{{IPaddr|0.0.0.0|0}}
| +{{IPaddr|255.255.255.255|}}
|{{IPaddr|0.0.0.0|}}
| style="text-align:right;" |4,294,967,296
| style="text-align:right;" |2<sup>32</sup>
| style="text-align:right;" |256 A
|
| Entire IPv4 Internet, [[default route]].
|}

In routed subnets larger than {{IPaddr||31}} or {{IPaddr||32}}, the number of available host addresses is usually reduced by two, namely the largest address, which is reserved as the [[broadcast address]], and the smallest address, which identifies the network itself.<ref name="RFC 922">{{cite IETF |rfc=922 |title=Broadcasting Internet Datagrams in the Presence of Subnets |editor=J. Mogul |date=October 1984 |section=7}}</ref><ref name="RFC 1812">{{cite IETF |rfc=1812 |title=Requirements for IP Version 4 Routers |editor=F. Baker |date=June 1995 |section=4.2.3.1}}</ref>

In such usage, a {{IPaddr||31}} network, with one binary digit in the host identifier, is unusable, as such a subnet would provide no available host addresses after this reduction. {{IETF RFC|3021}} creates an exception to the "host all ones" and "host all zeros" rules to make {{IPaddr||31}} networks usable for point-to-point links. {{IPaddr||32}} addresses (single-host network) must be accessed by explicit routing rules, as there is no address available for a gateway.