How to change ip addressing format from CIDR notation to netmask and vice versa?

Shell Programming and Scripting
1

Hi all, I would appreciate if someone could share how to convert CIDR notation to netmask and vice versa.
The value below is just an example. it could be different numbers/ip addresses.

Initial Output, let say file1.txt

Final Output, let say file2.txt

2

What's your shell? What's your system?

CIDR to netmask is straightforward if tedious -- just binary math. Shells aren't generally that good at it but modern BASH has large enough numbers to handle this.

#!/bin/bash

while IFS="/" read IP S
do
        M=$(( 0xffffffff ^ ((1 << (32-S)) -1) ))
        echo "$IP netmask $(( (M>>24) & 0xff )).$(( (M>>16) & 0xff )).$(( (M>>8) & 0xff )).$(( M & 0xff ))"
done

If you have an older bash, it may generate negative values due to overflowing its 32-bit integers.

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3

Awesome! Thanks Corona688 for the script. It's not what I'm looking for, but the script is good.
Here is my actual question, I've rephrased it and created separate topic for this.

4

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