DNS is the Domain Name System. Every website address connects to a domain name; for example, www.computerites.com and google.com are both domain names. IP addresses are actually what we are connecting to on the Internet. DNS was produced in order to make webpages easier to access, for each one server IP address assigned a domain name. A DNS server error happens when one can't access the IP address that corresponds with the domain name.

DNS server errors may be caused by the DNS server being switched off or having undergone a harmful error of some kind. Links between the  DNS server and your computers may be set incorrectly. A complete DNS server failure is possible, but check for other likely problems before replacing your server.

There are a lot of the servers to translate the domain name and the IP address, depending on the line and geographic location of internet users, there are different servers to provide DNS resolution service.

If ISP provider assigns you a dedicated IP address, then please fill in the correct IP address and DNS server address provided by your ISP.ISP provider assigns dynamic IP addresses, so no need to fill in the DNS server address.If the ISP provider DNS server error can not correct domain name → IP addresses conversion,you can use the Google DNS sever address .

And any computer virus may cause the DNS sever errors,then you need to kill the viruses.Now you will solve the 90% problems.

This address is http://www.computerites.com/internet-skills/2012/12/solution-to-fix-dns-server-error-635.html