A forgiving CIDR calculator.

cidr

#!/usr/bin/env bash

bin() {
  local str num=$1

  while (( num > 0 )); do
    str=$(( num % 2 ))$str
    (( num /= 2 ))
  done

  while (( ${#str} < 8 )); do
    str=0$str
  done

  printf %s "$str"
}

dec() {
  local pow=$(( ${#1} - 1 )) num=0

  for (( count = 0; count < ${#1}; ++count )); do
    (( num += ${1:count:1} * (2 ** pow) ))
    (( pow -= 1 ))
  done

  printf %d "$num"
}

2ip() {
  local ip=( ) count

  for count in {0..3}; do
    ip[$count]=$(dec "${1:count * 8:8}")
  done

  printf %s.%s.%s.%s "${ip[@]}"
}

if (( ! $# )); then
  printf 'usage: %s ip/cidr\n' "$0"
  exit 1
fi

IFS=/ read -r ip size <<< "$1"
IFS=. read -ra octets <<< "$ip"

for (( count = 0; count < 4; ++count )); do
  octets[$count]=$(bin "${octets[$count]}")
done

IFS= ip=${octets[*]} network=${ip:0:size} broadcast=${ip:0:size}

while (( ${#network} < 32 )); do
  network+=0
  broadcast+=1
done

for (( count=0; count < size; ++count )); do
  mask+=1
done

for (( count=32 - size; count < 32; ++count )); do
  mask+=0
done

printf 'subnet mask: %s\n' "$(2ip "$mask")"
printf 'range: %s - %s\n' "$(2ip "$network")" "$(2ip "$broadcast")"
printf 'total assignable hosts: %d\n' "$(( (2 ** (32 - size)) - 2 ))"