Computing & ElectronicsMathematics

CIDR

https://cidr.xyz/ example

Great resources are available to let the user understand the CIDR notation (let’s call it the modern way to specify a submask).

When you get the logic behind it, probably the best is this online calculator cidr.xyz.

To understand the meaning, use this tutorial.

For me worked having a visual comparative, like the following.

My way

Considering this notation:
s = significat digit
n = not significat digit
w = whatever

and this CIDR notation: 192.168.0.0/24

The evaluation to be conduct (and the result) is:

binary base IP   11000000.10101000.00000000.00000000
mask (i.e. /24)  ssssssss.ssssssss.ssssssss.nnnnnnnn
result (generic) 11000000.10101000.00000000.wwwwwwww

cases                                                    result 
result (generic) 11000000.10101000.00000000.wwwwwwww
192.168.0.0      11000000.10101000.00000000.00000000 --> admitted 
192.168.0.1      11000000.10101000.00000000.00000001 --> admitted
192.168.0.255    11000000.10101000.00000000.11111111 --> admitted
192.168.3.15     11000000.10101000.00000011.00001111 --> blocked

 

Consider now this other CIDR notation: 192.168.0.0/21

The evaluation to be conduct (and the result) is:

binary base IP   11000000.10101000.00000000.00000000
mask (i.e. /21)  ssssssss.ssssssss.sssssnnn.nnnnnnnn
result (generic) 11000000.10101000.00000www.wwwwwwww

cases                                                    result 
result (generic) 11000000.10101000.00000www.wwwwwwww
192.168.0.0      11000000.10101000.00000000.00000000 --> admitted 
192.168.0.1      11000000.10101000.00000000.00000001 --> admitted
192.168.0.255    11000000.10101000.00000000.11111111 --> admitted
192.168.3.15     11000000.10101000.00000011.00001111 --> admitted

 

 

Of course, I almost forgot to say that, formally, CIDR means classless inter domain routing.

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