DNS, for Domain Name Service, acts as a look-up table that allows the correct servers to be contacted when a user enters a URL into a Web browser. This somewhat transparent service also provides other features that are commonly used by webmasters to organize their data infrastructure.
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DNS runs on DNS servers. When a user enters a URL, such as www.google.com, into a Web browser the request is not directly sent to the Google servers. Instead, the request goes to a DNS server, which uses a look-up table to determine several pieces of information, most importantly the IP address of the website that is being requested. It then forwards this request to the proper servers and returns the information requested to the user's Web browser.
Domain Name System
The DNS server looks at three primary pieces of information, starting with the top level domain. The top-level domain is denoted by suffixes such as .com, .org, and .gov. Once the top-dlevel domain is established, the second-level domain is analyzed. For example, the URL www.google.com possesses a top-level domain of .com and the second-level domain name google. The second-level domain is usually referred to simply as a domain name. Finally, the DNS server resolves the third-level domain, or subdomain, which is the "www" portion of the URL.
Features of Subdomains
Aside from the "www" subdomain zone, other subdomains are also worth noting. For example, subdomains such as "pop" "irc" and "aliases" exist. Each subdomain represents a different service that may be accessed on the server. For example, "pop" is used for email communications. The use of the DNS server to resolve the IP addresses of these different services allows for complex network architectures to be implemented. Despite being under the same domain name, these different services may be hosted on different machines or different geographical locations. This also allows a level of redundancy when using aliases, in case the primary domain server goes down.
DNS servers allow standard Internet users to use Internet resources without having to remember port numbers and IP addresses. Even similar services, such as different areas of the website, may be hosted at different IP addresses for security reasons. This allows users to memorize simple URL addresses as opposed to complex, nonintuitive lists of IP addresses and port numbers. This also allows private servers made by home users to be freely available yet somewhat shielded from having their IP address publicly known.