Subnet mask to CIDR
The CIDR reference on this site is keyed by prefix length, which is the wrong way round when what you have in front of you is a mask. This is the same 33 networks, keyed by the mask.
Converting between the two needs no arithmetic: the prefix is the number of one-bits in the mask. 255.255.255.0 is twenty-four ones and eight zeros, so it is /24.
| Subnet mask | CIDR | Addresses | Usable | Wildcard |
|---|---|---|---|---|
| 0.0.0.0 | /0 | 4,294,967,296 | 4,294,967,294 | 255.255.255.255 |
| 128.0.0.0 | /1 | 2,147,483,648 | 2,147,483,646 | 127.255.255.255 |
| 192.0.0.0 | /2 | 1,073,741,824 | 1,073,741,822 | 63.255.255.255 |
| 224.0.0.0 | /3 | 536,870,912 | 536,870,910 | 31.255.255.255 |
| 240.0.0.0 | /4 | 268,435,456 | 268,435,454 | 15.255.255.255 |
| 248.0.0.0 | /5 | 134,217,728 | 134,217,726 | 7.255.255.255 |
| 252.0.0.0 | /6 | 67,108,864 | 67,108,862 | 3.255.255.255 |
| 254.0.0.0 | /7 | 33,554,432 | 33,554,430 | 1.255.255.255 |
| 255.0.0.0 | /8 | 16,777,216 | 16,777,214 | 0.255.255.255 |
| 255.128.0.0 | /9 | 8,388,608 | 8,388,606 | 0.127.255.255 |
| 255.192.0.0 | /10 | 4,194,304 | 4,194,302 | 0.63.255.255 |
| 255.224.0.0 | /11 | 2,097,152 | 2,097,150 | 0.31.255.255 |
| 255.240.0.0 | /12 | 1,048,576 | 1,048,574 | 0.15.255.255 |
| 255.248.0.0 | /13 | 524,288 | 524,286 | 0.7.255.255 |
| 255.252.0.0 | /14 | 262,144 | 262,142 | 0.3.255.255 |
| 255.254.0.0 | /15 | 131,072 | 131,070 | 0.1.255.255 |
| 255.255.0.0 | /16 | 65,536 | 65,534 | 0.0.255.255 |
| 255.255.128.0 | /17 | 32,768 | 32,766 | 0.0.127.255 |
| 255.255.192.0 | /18 | 16,384 | 16,382 | 0.0.63.255 |
| 255.255.224.0 | /19 | 8,192 | 8,190 | 0.0.31.255 |
| 255.255.240.0 | /20 | 4,096 | 4,094 | 0.0.15.255 |
| 255.255.248.0 | /21 | 2,048 | 2,046 | 0.0.7.255 |
| 255.255.252.0 | /22 | 1,024 | 1,022 | 0.0.3.255 |
| 255.255.254.0 | /23 | 512 | 510 | 0.0.1.255 |
| 255.255.255.0 | /24 | 256 | 254 | 0.0.0.255 |
| 255.255.255.128 | /25 | 128 | 126 | 0.0.0.127 |
| 255.255.255.192 | /26 | 64 | 62 | 0.0.0.63 |
| 255.255.255.224 | /27 | 32 | 30 | 0.0.0.31 |
| 255.255.255.240 | /28 | 16 | 14 | 0.0.0.15 |
| 255.255.255.248 | /29 | 8 | 6 | 0.0.0.7 |
| 255.255.255.252 | /30 | 4 | 2 | 0.0.0.3 |
| 255.255.255.254 | /31 | 2 | 2 | 0.0.0.1 |
| 255.255.255.255 | /32 | 1 | 1 | 0.0.0.0 |