Pings and individual server to update the arp table.
function pingServer (address) {
  return new Promise(function (resolve) {
    var socket = new net.Socket()
    socket.setTimeout(1000, close)
    socket.connect(80, address, close)
    socket.once('error', close)

    function close () {
      socket.destroy()
      resolve(address)
    }
  })
}