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

Cant help you finding out if the IP is in use, but it would be a good time
to start a spreadsheet with all IP address's on it. DHCP Ranges and
individual hard assignments. Will make it much easier next time.

Bob

Bob Crothers
www.BJsBariatrics.Com
------------------------------------------
If you think dogs can't count, try putting three dog biscuits in your
pocket and then giving Fido only two of them.
~-Phil Pastoret


On Thu, Aug 14, 2014 at 3:43 PM, David Gibbs <david@xxxxxxxxxxxx> wrote:

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

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