Lack of progress report:
I had a nearby friend with an identical cable modem / router and also using Comcast try shorpy. He had no problem.
I suspected the DNS I use so I found the IP address of shorpy (63.247.140.19) and entered that in my browser. I still can't access shorpy. So then I looked in my cable modem / router for blocked IP addresses. There aren't any, at least where I looked (Access Restrictions).
I've tried turning the firewall off. That doesn't help.
I doubt that it's the MTU setting, since I've tried a number of computers using this cable modem/router and all had trouble. One of them has WiFi and works with the neighbor's unencrypted Access Point, but CANNOT access shorpy or its sub-sites through my cable modem / router instead of WiFi.
If the modem / router has a MTU, I haven't tried that, but why would it suddenly change last week?
I continue to be stumped.
seacue