docker-mailserver/target/scripts/helpers/network.sh

#!/bin/bash

function _mask_ip_digit() {
  if [[ ${1} -ge 8 ]]; then
    MASK=255
  elif [[ ${1} -le 0 ]]; then
    MASK=0
  else
    VALUES=(0 128 192 224 240 248 252 254 255)
    MASK=${VALUES[${1}]}
  fi

  local DVAL=${2}
  ((DVAL&=MASK))

  echo "${DVAL}"
}

# Transforms a specific IP with CIDR suffix
# like 1.2.3.4/16 to subnet with cidr suffix
# like 1.2.0.0/16.
# Assumes correct IP and subnet are provided.
function _sanitize_ipv4_to_subnet_cidr() {
  local DIGIT_PREFIX_LENGTH="${1#*/}"

  declare -a MASKED_DIGITS DIGITS
  IFS='.' ; read -r -a DIGITS < <(echo "${1%%/*}") ; unset IFS

  for ((i = 0 ; i < 4 ; i++))
  do
    MASKED_DIGITS[i]=$(_mask_ip_digit "${DIGIT_PREFIX_LENGTH}" "${DIGITS[i]}")
    DIGIT_PREFIX_LENGTH=$((DIGIT_PREFIX_LENGTH - 8))
  done

  echo "${MASKED_DIGITS[0]}.${MASKED_DIGITS[1]}.${MASKED_DIGITS[2]}.${MASKED_DIGITS[3]}/${1#*/}"
}