Who is your ISP (Internet Service Provider)?
You type a name like "gunsinternational.com" in your browser and a request is forwarded to a Domain Name Server (DNS) to translate what you typed into the IP address the website name is associated with. For example, 69.63.128.230 is the address for GunsInternaitonal.com. The Domain Name Servers have a file with a list of domain names and IP (numerical) addresses. When the IP address is identified, your browser is forwarded to that IP address. You don't see it happening because the website domain name still shows in your browser address bar.
If the particular Domain Name Server your system forwards requests to (usually a primary and an alternate) is having problems you will see a DNS error message. Often you will see something like "address could not be found", "Unable to Find...", "DNS Server Not Responding", or something similar.
Your ISP will have things set up to use their default DNS servers. CenturyLink, my ISP, has been having intermittent DNS problems for weeks. I tired of it and configured my system to forward to a pair of public DNS servers.
Gibson Research has a really nice benchmark tool for determining the best DNS servers for your location. It is totally free:
https://www.grc.com/dns/benchmark.htm If nothing else you can see how your currently configured DNS servers stack up against others.
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