Works on Mac. TODO: Linux - Ubuntu et al.
function getArpTable (callback) {
  var arp = spawn('arp', ['-a'])
  var arpStr = []

  arp.stdout.setEncoding('utf8')
  arp.stdout.on('data', function (data) {
    arpStr.push(data)
  })
  arp.on('close', function (x) {
    callback(null, parseArpTable(arpStr))
    // arp.kill();
  })
}