can ping, but no internet services

I've lost internet connection on my MacBook Pro 13" mid-2010, 2.66 GHz, High Sierra 10.13.4. From the Terminal, I can ping both IP addresses and domain names over two different WiFi networks (home & work), but Mail, several browsers, Dropbox, etc., don't connect to the Internet. I noticed the problem Monday, things were working fine last week, and I haven't done any upgrades or installations (that I know of) in the past week. Yesterday I did a System Recovery from my Time Capsule from 10 days ago, thinking it was some recent update to some app or system file, but the problem persists. I've searched the web for similar problems, but none of the solutions I found apply (most were WiFi or DNS issues). Any ideas?