DNS servers resolve Domain Names to IP Addresses. If they go wrong, you can't access some sites.
What do you do if suddenly, some websites don't seem to want to load on your computer? I have been having this trouble for a week now, and the websites are all for the United States Patent & Trademark Office.
I called the USPTO electronic business center helpdesk, and they are usually on top of things. They indicated a lot of people using AT&T or UVERSE were having trouble accessing the site. They suggested that AT&T's DNS server was redirecting website requests improperly.
I called AT&T and they tried to help (three hours on the phone or in chat) but could not resolve the issue. Sometimes "flushing" the DNS cache (c:ipconfig /flushdns) sometimes would help, as would clearing cookies. But this only allowed access to some USPTO sites, and not all, and the next day, the problem would re-emerge.
The error message you get, depends on which browser you are using:
Explorer: This website cannot be displayedFirefox: Server not found. Firefox can't find the server at www.xyz.comChrome: This webpage is not available. The server at www.xyz.com can't be found, because DNS lookup failed. DNS is the network service that translates a website's name to its Internet address. This error is most often caused by having no connection to the internet or a misconfigured network. It can also be caused by an unresponsive DNS server or a firewall preventing Google Chrome from accesing the network. Error code: DNS_PROBE_FINISHED_NXDOMAIN
This latter error message (Chrome is more helpful than Firefox or Explorer in providing details) lead me to a Google Search which found some useful help.
This site has some helpful information:
It seems like you are having DNS problems. First thing I would do is try clearing your DNS cache:
If that does not help, then try changing your DNS servers temporarily to Google's (22.214.171.124 and 126.96.36.199) just to make sure that the DNS server that you are using is not having problems.
To do this on windows 7:
Well, I tried this, and it works. By using Google's DNS server instead of AT&T's, I can access the USPTO websites again. Damn you Google, for being so helpful! AT&T gets a "nice try" award, but their help desk people could not figure it out.