Friday, October 22, 2010

Domain Name Servers

  • Rarely do applications deal directly with IP addresses
    • a hierarchical system of domain names can be used instead
    • top level domains: edu, com, gov, org, net, …
    • commonly: hostname.subdomain.domain  (possibly many sub-domains)
    • e.g., in.news.yahoo.com
Domain names serve as “listings” in a “phone book” for the internet, i.e. when you enter a domain name in a web browser, a “look up” is done to find the corresponding IP address. Domain names also simplify things in that if the physical network changes (e.g. IP addresses are modified), the domain names can remain the same while the “phone book” entries are updated with the new IP address.

A domain name server (DNS) is a machine that keeps a table of names and corresponding IP addresses (i.e. this performs the “look up” to match domain names to their corresponding IP address). There are 13 root servers in the world.
While there are only 13 root servers, there are multiple physical servers (that mirror them) that operate in different geographic locations to provide reliable service in case of hardware failures.


  • When an application specifies a host name,
    • go to local domain name server and try lookup in its stored cache
    • if not stored there, then local DNS requests address from a root server
    • root server determines appropriate name server & forwards request
Famous Domains Name Hostings:
    • yahoo domains and web hosting
    • google domains

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