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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