CIDR Conversion Table
| Prefix Length | Netmask | Size (Bits) | Size (Addresses) |
|---|---|---|---|
| /0 | 0.0.0.0 | 32 | 4,294,967,296 (Entire Internet) |
| /1 | 128.0.0.0 | 31 | 2,147,483,648 |
| /2 | 192.0.0.0 | 30 | 1,073,741,824 |
| /3 | 224.0.0.0 | 29 | 536,870,912 |
| /4 | 240.0.0.0 | 28 | 268,435,456 |
| /5 | 248.0.0.0 | 27 | 143,217,728 |
| /6 | 252.0.0.0 | 26 | 67,108,864 |
| /7 | 254.0.0.0 | 25 | 33,554,432 |
| /8 | 255.0.0.0 | 24 | 16,777,216 (Class A) |
| /9 | 255.128.0.0 | 23 | 8,388,608 |
| /10 | 255.192.0.0 | 22 | 4,194,304 |
| /11 | 255.224.0.0 | 21 | 2,097,152 |
| /12 | 255.240.0.0 | 20 | 1,048,576 |
| /13 | 255.248.0.0 | 19 | 524,288 |
| /14 | 255.252.0.0 | 18 | 262,144 |
| /15 | 255.254.0.0 | 17 | 131,072 |
| /16 | 255.255.0.0 | 16 | 65,536 (Class B) |
| /17 | 255.255.128.0 | 15 | 32,768 |
| /18 | 255.255.192.0 | 14 | 16,384 |
| /19 | 255.255.224.0 | 13 | 8192 |
| /20 | 255.255.240.0 | 12 | 4096 |
| /21 | 255.255.248.0 | 11 | 2048 |
| /22 | 255.255.252.0 | 10 | 1024 |
| /23 | 255.255.254.0 | 9 | 512 |
| /24 | 255.255.255.0 | 8 | 256 (Class C) |
| /25 | 255.255.255.128 | 7 | 128 |
| /26 | 255.255.255.192 | 6 | 64 |
| /27 | 255.255.255.224 | 5 | 32 |
| /28 | 255.255.255.240 | 4 | 16 |
| /29 | 255.255.255.248 | 3 | 8 |
| /30 | 255.255.255.252 | 2 | 4 |
| /31 | 255.255.255.254 | 1 | 2 |
| /32 | 255.255.255.255 | 0 | 1 |
Note that /31 is not usually a useful subnet size. After you take out the network and broadcast addresses, there is nothing left. The only case where a /31 would be useful is if you were going to further subdivide the block into two /32's.