So, this morning, the house server needed restarting. Typically, it spontaneously rebooted about five minutes after being manually restated - ghod knows why. But my PC and the bellinghwoman's both got their net connectivity (nettivity?) back.
And of course, at work, the same thing.
The servers are all on UPSes. Even so, one needed a hard reset, and another one was unreachable from all other machines on the network - even from itself - though it could see all the others.