IP Application Services

Understanding the Domain Name System

Cisco - Understanding the Domain Name System

Document ID: 12683

Updated: Oct 26, 2005



This document discusses the Domain Name System.



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The Domain Name System (DNS) is the system in the Internet that maps names of objects (usually host names) into IP numbers or other resource record values. The name space of the Internet is divided into domains, and the responsibility for managing names within each domain is delegated, typically to systems within each domain.

For example, all Internet systems that belong to the University of Arizona have names within the domain. The Internet's root name servers delegate the responsibility for managing the name space to a name server system operated by University of Arizona's CCIT Telecom (a system which also happens to be called with the IP addresses and

The Telecom name server can, in turn, delegate portions of the name space to departmental name servers on campus. By this system, the department gains a measure of autonomy in inventing and managing the names within its subdomain. For example, some or all of the subdomains of can be nameserved by various departments (such as Computer Science, Math, or Physics).

In addition to the Internet being divided namewise into domains and subdomains, such as for University of Arizona and for Apple Computer, it is divided numberwise into networks and subnets, such as or for University of Arizona and Apple, respectively. The namewise layout of the Internet tracks administrative responsibility (ownership), while the numberwise layout tracks physical topology.

There is no necessary relationship between the name(s) of an object in the Internet and its number(s). For example, the network physically resides at the University of Arizona. However, if a machine that belongs to Apple were to be plugged into the University of Arizona network, its name would still be, even though its number would be In this case, however, Apple and the University of Arizona would share nameservice responsibility for this system: Apple for the name-to-number nameservice, and University of Arizona for the number-to-name nameservice.

The primary job that DNS performs is to map between names and numbers. Most importantly, it must provide the translation from host names to IP addresses, so that applications can effect a network connection from a command such as ftp Also, DNS must map from IP addresses back to names in order to provide some level of authentication, as with the r commands.

Reverse mapping from IP addresses to host names is performed under the auspices of the IN-ADDR.ARPA pseudo-domain. Because the order of significance in the naming system is highest on the right, the notation for addresses is reversed. Therefore, the DNS entry for the IP address is given as

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Updated: Oct 26, 2005
Document ID: 12683