Noted at https://www.wikihow.com/Flush-DNS
After this it gets technical. I need you to use NSLOOKUP and you to compare the answer to a working PC.
My Ethernet-connected desktop can resolve only about 30% of the sites that I try to visit. The problem is just with my desktop computer. I don't have any problems If I plug the desktop's Cat-5 cable into my laptop, so it is not a problem with my ISP (AT&T U-verse). My desktop is running Win 10 Pro 64. 10.0.17134
Some of the sites I CAN access include:
google.com, att.com, facebook.com (no sound), wikipedia.com, youtube.com (no sound), CDC.com, NIH.com, mozilla.org.
Some sites I CAN'T access include:
yelp, newegg.com, ebay.com, amazon.com, my Yahoo mail server, download.mozilla.org, and a whole bunch of various other sites.
Note that I CAN access mozilla.org, but I CAN'T access download.mozilla.org
The message I get for about 70% of the sites I try is:
"We can't connect to the server at xxx.xxx.xxx"
These are the things I have tried that have not worked:
1. PING - I can't PING anything, not even my own router or NIC. I get no response when I PING. It just sits there and does nothing. This may be the most important clue to the problem. My other computers can PING anything with no problem.
2. It is not within Firefox or Internet Explorer. They both give identical results or non-results.
3. Multiple reboots don't help.
4. I ran System File Checker and it came back clean.
5. I changed the IPv4 DNS addresses on the Ethernet card to Google's 18.104.22.168 and 22.214.171.124. I also changed the IPv6 DNS addresses to Google's.
6. I did a "Restore Firewall to Default" in Windows Defender. That didn't help.
7. Temporarily turning off Windows Defender Firewall didn't help either.
There is something wrong with this particular computer but I can't figure out what it is.