The browser reports the error closest to what it was doing at the time - host not found? Well, the network was reliable enough to reach a dns server that returned that the lack of address for a name. But if the dns server itself can’t come reached, it’s some sort of network error between you and that server. The typical way to diagnose that kind of problem is to perform all the steps yourself - can I ping the dns server address? Can I resolve this host with that dns server? What about a different dns server, maybe that particular name is being excluded because of corporate policy. The command line tools ping, traceroute and dig are useful if you want to get into it.