Understanding CIDR Subnet Mask Notation

pfSense uses CIDR (Classless Inter-Domain Routing) notation rather than the common subnet mask 255.x.x.x when configuring addresses and networks. Refer to the following Table CIDR Subnet Table: to find the CIDR equivalent of a decimal subnet mask.

CIDR Subnet Table
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 1024 1022 4
255.255.248.0 /21 2048 2046 8
255.255.240.0 /20 4096 4094 16
255.255.224.0 /19 8192 8190 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 1024
255.248.0.0 /13 524,288 524,286 2048
255.240.0.0 /12 1,048,576 1,048,574 4096
255.224.0 0 /11 2,097,152 2,097,150 8192
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 1,048,576
240.0.0.0 /4 268,435,456 268,435,454 2,097,152
224.0.0.0 /3 536,870,912 536,870,910 4,194,304
192.0.0.0 /2 1,073,741,824 1,073,741,822 8,388,608
128.0.0.0 /1 2,147,483,648 2,147,483,646 16,777,216
0.0.0.0 /0 4,294,967,296 4,294,967,294 33,554,432

Note

The use of /31 networks is a special case defined by RFC 3021 where the two IP addresses in the subnet are usable for point-to-point links to conserve IPv4 address space. Not all operating systems support RFC 3021, so use it with caution. On systems that do not support RFC 3021, the subnet is unusable because the only two addresses defined by the subnet mask are the null route and broadcast and no usable host addresses.

pfSense 2.4.3-RELEASE-p1 supports the use of /31 networks for interfaces and Virtual IP addresses

So 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.