Re: Find out if an IP address is in use on the LAN

On 8/14/2014 1:55 PM, Jeff Crosby wrote:

It does not show up in arp -a

My guess is that the IP isn't actually up then.

Here's my rationale:

When you ping the IP, the system has to determine the MAC address in order to address the system (I think).

To do this is does a broadcast asking who has that IP. The system who has that IP _MUST_ respond (if it can).

Once the system responds to the broadcast with it's mac address, that address gets put in the arp table.

Once the mac address is in the arp table, the ping process can start ... although the system that responded to the broadcast doesn't have to respond to the ping.

So if you ping the IP and there is no entry in the arp table, then the system isn't able to respond.

At least that's my theory. :)

david