Ping alternates "request timed out" / "destination host unreachable" -why?

[Ichinisan]

I leave continuous pings going all the time. I've always noticed that when a host is down or doesn't exist, the ping results occasionally show "destination host unreachable" and then go back to showing "request timed out" again. What condition is changing to affect this during my ping?

The "reply" doesn't come from the host I tried to ping. It comes from some router between here and there. However, it counts as a successful ping reply in the statistics -- which is confusing / annoying. The replies typically come in groups -- as if some condition changed for a few seconds and then goes back to how it was before.

At the small ISP where I work, I am sometimes asked to allocate a static IP address for a customer. I like to do a quick ping before I finalize the assignment. A couple times over the years I've found that another customer was already using a static / manual IP address and it hadn't been properly documented, so I'd avoid using that address and avoid a conflict. There's no guarantee I'd get a ping reply if someone is using it, but I would definitely avoid using a particular address if I got ping replies from it.

In that case from the screenshot, I'm confident the IP address has not been used by anyone for years. What condition is changing during my continuous ping that causes me to receive different messages as the ping fails?

 

[Fardringle]

My understanding is that the main difference between the two messages is that "request timed out" means that the router/device giving the reply believes that it knows where the destination is supposed to be, but did not get a reply, and "destination host unreachable" means that no valid route could be found to the device, usually because it's an unused/unknown address.

As far as the response changing for the same ping, your ISP or something further up the route might be trying different routes so you are getting different replies from different routers somewhere along the path where some think they know the IP address (probably very old routing tables that haven't been purged) and some don't.