Let's take a look at an example where an ARP request fails.

What we'll do is start a new capture, and this time we will ping an address that does not exist on my network:

We will see Destination host unreachable show up. We will do that a couple of times. Since we are trying a ping, it's going to do that four times on a Windows system. If you're on Linux or macOS, it'll probably do that nonstop, depending on what you use.

Go ahead and stop that capture; we have plenty of information here:

We can see the request highlighted in the preceding screenshot: Who has 192.168.77.124?

We can see the target down in the Address Resolution Protocol (request) option. And we just keep requesting it over and over and over again. We're trying desperately to find who has that MAC address for IP 192.168.77.124, but it doesn't exist. It just keeps trying and trying, and it keeps timing out and failing; and that will show up in our results, as you saw in the command line, as unreachable.