By: Me1 Domain Name Servers (DNS)

By: Me2 Section 1: Overview Of DNS DNS is a name resolution protocol. It converts host names to IP addresses and vice-versa.

By: Me3 DNS Structure DNS is a a hierarchical database (tree) much like windows file system

By: Me4 On the internet the first branches coming out of the tree are the “top-level domains”. Examples are.gov being the government domain, and.com, a commercial domain.

By: Me5 Under the “top-level domains” there are other branches containing more domains, like aol.com and cisco.com.

By: Me6 You can see the tree in its natural environment.

By: Me7 DNS Host Names Host names are based on the tree structure of the DNS. Individual node names come first, followed by domain, and so on. Example: ganesan.calstatela.edu Ganesan is in the calstatela domain, which is in the edu domain, which is under the root domain of the internet.

By: Me8 DNS Delegation Each domain has it’s own name server. Name servers give host name information to all hosts and sub- domains within their domains. The microsoft.com domain, for example has a name server which stores all of this information.

By: Me9 However, authority for a subdomain, such as nerds.microsoft.com, can be passed to a name server that has authority for that subdomain. When a name resolution request comes to the microsoft.com name server, it just passes the request off to the nerds.microsoft.com name server. Through this we can see that DNS is truly distributed across the Internet, with each domain maintaining only the information that is pertinent to that domain.

By: Me10 Name Resolution As a result of DNS being distributed across domains, when a name server receives a request for name resolution for a host that is outside of its own domain, it may not have the address information for that host. Because DNS is hierarchical, it does not need that information, the name server just needs to know how to access the root name server.

By: Me11 The name server forwards the name resolution request to the root name server, which then delegates the request to the appropriate domain beneath it, and this process continues until a name server which has address information for the host is reached, and the information is retrieved.

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By: Me13 Caching In order to reduce network traffic, which is quite important on the net, DNS’ use caching. (Like when you go the bank to get your money) Whenever a name server receives address information for another host or domain, it stores that information for a specified period of time.

By: Me14 That way, if another name resolution request for that host or domain is received, the name server has the address information ready, and does not need to send another inquiry across the Internet.

By: Me15 The length of time address information is stored on the name server is determined by the Time-To-Live (TTL) value entered in the domain Start of Authority (SOA) resource record.

By: Me16 Configuring DNS server Name Servers store info about their domains in several different “resource records”. Each resource record stores different info about their domain and host of the domain. Resource Records are usually text files stored on the domain name server. Some domain name browsers let you edit the records through a gui.

By: Me17 For more information on Domain Name Servers, go to and type in DNS.

By: Me18 Or you can go here:

By: Me19 END OF MODULE