Network CIDR Notation
Many firewalls and network devices use CIDR (Classless Inter-Domain Routing) notation rather than the common subnet mask 255.x.x.x when configuring addresses and networks. The table below gives the CIDR equivalent of each decimal subnet mask.
| Subnet Mask | CIDR Prefix | Total IP Addresses | Usable IP Addresses | Number of /24 networks |
|---|---|---|---|---|
| 255.255.255.255 | /32 | 1 | 1 | 1/256th |
| 255.255.255.254 | /31 | 2 | 2* | 1/128th |
| 255.255.255.252 | /30 | 4 | 2 | 1/64th |
| 255.255.255.248 | /29 | 8 | 6 | 1/32nd |
| 255.255.255.240 | /28 | 16 | 14 | 1/16th |
| 255.255.255.224 | /27 | 32 | 30 | 1/8th |
| 255.255.255.192 | /26 | 64 | 62 | 1/4th |
| 255.255.255.128 | /25 | 128 | 126 | 1 half |
| 255.255.255.0 | /24 | 256 | 254 | 1 |
| 255.255.254.0 | /23 | 512 | 510 | 2 |
| 255.255.252.0 | /22 | 1,024 | 1,022 | 4 |
| 255.255.248.0 | /21 | 2,048 | 2,046 | 8 |
| 255.255.240.0 | /20 | 4,096 | 4,094 | 16 |
| 255.255.224.0 | /19 | 8,192 | 8,190 | 32 |
| 255.255.192.0 | /18 | 16,384 | 16,382 | 64 |
| 255.255.128.0 | /17 | 32,768 | 32,766 | 128 |
| 255.255.0.0 | /16 | 65,536 | 65,534 | 256 |
| 255.254.0.0 | /15 | 131,072 | 131,070 | 512 |
| 255.252.0.0 | /14 | 262,144 | 262,142 | 1,024 |
| 255.248.0.0 | /13 | 524,288 | 524,286 | 2,048 |
| 255.240.0.0 | /12 | 1,048,576 | 1,048,574 | 4,096 |
| 255.224.0.0 | /11 | 2,097,152 | 2,097,150 | 8,192 |
| 255.192.0.0 | /10 | 4,194,304 | 4,194,302 | 16,384 |
| 255.128.0.0 | /9 | 8,388,608 | 8,388,606 | 32,768 |
| 255.0.0.0 | /8 | 16,777,216 | 16,777,214 | 65,536 |
| 254.0.0.0 | /7 | 33,554,432 | 33,554,430 | 131,072 |
| 252.0.0.0 | /6 | 67,108,864 | 67,108,862 | 262,144 |
| 248.0.0.0 | /5 | 134,217,728 | 134,217,726 | 524,288 |
| 240.0.0.0 | /4 | 268,435,456 | 268,435,454 | 1,048,576 |
| 224.0.0.0 | /3 | 536,870,912 | 536,870,910 | 2,097,152 |
| 192.0.0.0 | /2 | 1,073,741,824 | 1,073,741,822 | 4,194,304 |
| 128.0.0.0 | /1 | 2,147,483,648 | 2,147,483,646 | 8,388,608 |
| 0.0.0.0 | /0 | 4,294,967,296 | 4,294,967,294 | 16,777,216 |
* On a /31 network both addresses are usable, since it is a point-to-point link (RFC 3021).
Where do CIDR numbers come from?
The CIDR number comes from the number of ones in the subnet mask when converted to binary.
The common subnet mask 255.255.255.0 is 11111111.11111111.11111111.00000000 in binary. This adds up to 24 ones, or /24 (pronounced «slash twenty four»).
A subnet mask of 255.255.255.192 is 11111111.11111111.11111111.11000000 in binary, or 26 ones, hence /26.
Source: Netgate pfSense documentation, Understanding CIDR Subnet Mask Notation.