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Network
 Understanding CIDR Subnet Mask Notation

The modern IP network uses a subnet mask format that you may not be familiar with. Rather than the common 255.x.x.x, it uses CIDR (Classless InterDomain Routing) notation.

CIDR Table
You can refer to the following table to find the CIDR equivalent of your subnet mask.

Subnet Mask  CIDR Prefix Total IP's Usable IP's Number of
Class C networks
255.255.255.255 /32 1 1 1/256th
255.255.255.254 /31 2 0 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 1/2half
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 8,19 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 /15 131,072 131,07 512
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 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
\
So where do these CIDR numbers come from anyway?

The CIDR number comes from the number of 1's 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 1's, or /24 (pronounced 'slash twenty four').

A subnet mask of 255.255.255.192 is 11111111.11111111.11111111.11000000 in binary, or 26 1's, hence a /26.

And so on ...

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