How do I find a remote hosts's MAC address if I have an IP address?

[vszabad]

Hello everyone,

Recently, a user had some issues with an application and after I have obtained his IP address and logged onto the router I typed the following command: #show ip arp vrf "vrf" "ip"  , but it had no output. I understand that this is because the user's IP sits in another subnet. My question would be - How can I find this hosts, that sits in another subnet, MAC address from the router? 

Many thanks

[David Ruess]

Hello,

How did you get the IP? If you asked the user then you could just as easily have them open a command prompt to give you the MAC address of the devices (assuming windows).

If it's in another subnet and since routers/L3 devices don't propagate MAC addresses I'm not sure if there is a command for that unless you are on the device where the subnet resides. In that case the command should work.

-David

[vszabad]

There is a switch that connects to both the router and the host. Is there a way to find the MAC address from the router. The host and the router are not in the same subnet. 

[Richard Burts]

There are things about this environment that we do not know and it complicates our effort to give good advice. You tell us "The host and the router are not in the same subnet." so this suggests that the switch providing connectivity is operating as a layer 3 switch (providing routing between subnets/vlans). If that is the case then the command show ip arp on the switch should show the IP address and its associated mac address.

[Leo Laohoo]

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@vszabad wrote:

How can I find this hosts, that sits in another subnet, MAC address from the router? 

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• Can the wired client ping it's own IP address? 
• Can the wired client ping the default gateway? 

Start with those two and let's see what turns up. 

If the ARP table does not have any entries then I am going to presume the following: 

• Client has "fallen off the network" and stopped talking. 
• IP or MAC address provided is incorrect. 

[Joseph W. Doherty]

Assuming you have access to that's PC's gateway device, I log onto that device and ping the IP in question.  Assuming there's a ping reply, I then look at the gateway's device's ARP table.

[Jitendra Kumar]

can you please share the network Diagram. that will to understand what is the packet flow..

Thanks,

Jitendra