CIDR from netmask

example.js

function hammingWeight(i) {
  i = i - ((i >> 1) & 0x55555555);
  i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
  return ((i + (i >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}

function netmaskToCIDR(netmask) {
  let chunks = netmask.split('.')
  if (chunks.length != 4) {
    return false;
  }
  
  let lastChunk = 255
  let acc = 0

  for (let i = 0; i < 4; i += 1) {
    let chunk = parseInt(chunks[i], 10)

    if (isNaN(chunk)) {
      return false
    }

    if (lastChunk < chunk) {
      return false
    }

    if (chunk === 0 || chunk === 255 || chunk === 254 || chunk === 252 || chunk === 248 || chunk === 240 || chunk === 224 || chunk === 192 || chunk === 128) {
      acc += hammingWeight(chunk)
      lastChunk = chunk
    } else {
      return false
    }   
  }

  return acc
}