OK - turned on the Mac, fired up Chrome and Safari, everything is working again. I know I powered down and restarted several times last night, so I don't know when the Interweb Fairy showed up to fix things.
Probably a bad negative DNS cache on the Mac after it didn’t get a reply from something, judging by how it stuck to the Mac like that.
Could have also been something stupid like the Mac trying to route to an IPv6 address with bad routes while the majority of us are on IPv4, but I’d have to go look and see if PoA publishes an IPv6 address from AWS where I believe it lives.
Look up “clear Mac DNS cache” with Google and do that command line procedure they detail in the top result if it happens again, and see if it stops immediately.
Also have seen two Macs go completely out to lunch on IP routing even with firewalls turned off and on and network adapters hard reset recently, at the office. Happens a while after the High Sierra upgrade and only a complete reboot was found to clear it.
Both machines using Apple’s crappy Ethernet dongle at least part time but switching to WiFi didn’t help. ARP table and routing table looked absolutely normal. Best guess is a kernel bug in the Ethernet single driver locking up the networking stack, or locking kernel memory for DMA and not releasing it, but no hints in the logs.
Sad to see Apple screwing up the rock solid underlying original BSD networking stack that can even smoke Linux in raw performance usually. (Netflix can’t run Linux on their streaming servers, the Linux network stack won’t stay stable. BSD to the rescue, as always. But Apple has hacked the hell out of their BSD original roots and it’s really not the same stack anymore.)