function normalize (mac) {
  let m = CISCO_MAC_ADDRESS_RE.exec(mac)
  if (m) {
    const halfwords = m.slice(1)
    mac = halfwords.map((halfword) => {
      return zeroFill(4, halfword)
    }).join('')
    return chunk(mac, 2).join(':').toUpperCase()
  }

  m = MAC_ADDRESS_RE.exec(mac)
  if (m) {
    const bytes = m.slice(1)
    return bytes
      .map(byte => zeroFill(2, byte))
      .join(':')
      .toUpperCase()
  }
}
Takes a MAC address in various formats:

     - 00:00:00:00:00:00,
     - 00-00-00-00-00-00,
     - 0000.0000.0000

 ... and returns it in the format 00:00:00:00:00:00.

@param  {string} mac
@return {string}