[Cialug] Basic Networking ARP and Routers

[Zachary Kotlarek]

On Jul 24, 2014, at 1:04 PM, Nick Graber <nick at graber.me> wrote:

> The Schneider PLC is using ARP outside of it's subnet and expecting a reply
> from the router with it's MAC to make a connection with the AB PLC I think.
> Should the router replay to such and inquiry or should the PLC do an ARP
> for the Default Gateway then send the packet to the Default Gateway?  It
> appears it is using ARP to try and discover the router that controls
> 172.20.87.255 and I'm not sure that is possible.


It looks like the Schneider thinks that 172.20.87.235 is link-local. Otherwise it would ARP for the router, not the destination.

My guess is the Schneider is not configured to use the router, either because the subnet information it has is wrong (maybe set to 0.0.0.0) or because it does not support IP routing (which sometimes happens on dumb equipment even though it pretends to speak IP).

It is possible to bridge the two networks in such a way that this works, but I’d start by ensuring that 172.20.87.235 doesn’t match the subnet on the Schneider.

	Zach

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