convert between cidr and netmask

convert_between_cidr_and_netmask.sh

#!/bin/bash

# src: https://stackoverflow.com/questions/20762575/explanation-of-convertor-of-cidr-to-netmask-in-linux-shell-netmask2cdir-and-cdir

mask2cidr ()
{
   # Assumes there's no "255." after a non-255 byte in the mask
   local x=${1##*255.}
   set -- 0^^^128^192^224^240^248^252^254^ $(( (${#1} - ${#x})*2 )) ${x%%.*}
   x=${1%%$3*}
   echo $(( $2 + (${#x}/4) ))
}

cidr2mask ()
{
   # Number of args to shift, 255..255, first non-255 byte, zeroes
   set -- $(( 5 - ($1 / 8) )) 255 255 255 255 $(( (255 << (8 - ($1 % 8))) & 255 )) 0 0 0
   [ $1 -gt 1 ] && shift $1 || shift
   echo ${1-0}.${2-0}.${3-0}.${4-0}
}

# cidr2mask 16 => 255.255.0.0
# mask2cidr 255.255.0.0 => 16