Just as a sneeze might be a warning that you've caught a cold, a DNS error is usually an early warning that you've completely lost network or Internet access. This error indicates that you've been unable to look up the IP address for a domain that you're trying to access.
Domain Name System
Though many people have not heard of the domain name system, the Internet would not run without it. The Internet consists of many domains, or websites, that are housed on interconnected servers. Every server has an IP address that is used to locate it on the Internet and to send Web page and other requests. We usually refer to domains by their domain names — for example, eHow.com — but computers need to know the IP address of the server that hosts the domain, such as 220.127.116.11. Special DNS servers bridge this gap by quickly translating domain names to IP addresses.
Every time your browser makes a request to a website, it silently requests the IP address of that site from a DNS server in the background. Because DNS lookups are a precursor to website visits, DNS errors are usually an early indication that you have connectivity problems. Standard network configurations require at least two DNS servers: a primary server and a backup DNS server in case the primary server fails. It's highly unlikely that a DNS error is the result of two or more simultaneous DNS server failures.
DNS Impact on Browsing Speed
Because DNS lookups happen frequently and continuously as you browse the Internet, the speed of those lookups can substantially impact how fast you perceive your Internet speed. If a primary DNS server fails and every lookup must go to the secondary DNS server, this significantly slows your Internet access — every DNS lookup must wait until the primary DNS server times out before the request is sent to the secondary DNS server. Even small changes to a DNS configuration, such as using a DNS server that's physically closer to you, can have an impact on your perception of the overall speed of the Internet.
Public DNS Servers
While you typically get your default DNS servers from your Internet service provider, Google, OpenDNS and other companies also provide public DNS servers that you can set as the DNS lookup devices for your network or computer. Google's public DNS servers purport to provide extremely fast lookup results, and OpenDNS servers provide fast lookups and the ability to implement parental controls by returning the IP address of a Web page that informs you a website has been blocked when your computer requests the IP address of a prohibited domain. Many wireless routers also provide some DNS functionality to speed up the process. Your router might run a type of DNS server called a relay. The relay stores recent lookups in memory so the router instantly responds to a DNS request for the IP address of a website that you or someone else on the network already visited. It only has to relay brand new requests to a public DNS server, and it adds those results to its local cache.