title: arp -a does not list all devices
body: <p>I have an app which checks whether a certain IP is on the local network. The IP belongs to a raspberry pi working as a homeautomation device.
  Running the command</p>
  
  <pre><code>arp -a
  </code></pre>
  
  <p>from terminal or from within my app I get a list of all devices on the local network, but my raspberry pi is not always listed. After pinging the device, it appears on the list every time.
  Any ideas what is going wrong? </p>