The domain name system (DNS) is the system in the Internet that maps names of objects (most usually host names) into IP numbers or other resource record values. The namespace 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 belonging to the University of Arizona (U of A) have names within the arizona.edu domain. The Internet's root nameservers delegate the responsibility for managing the arizona.edu namespace to a name server system operated by U of A CCIT Telecom (a system which also happens to be called arizona.edu–IP addresses 128.196.128.233 and 128.196.128.234.) The Telecom nameserver can, in turn, delegate portions of the arizona.edu namespace 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 arizona.edu can be nameserved by various departments (such as As, CS, Math, or Physics.) In addition to the Internet being divided namewise into domains and subdomains, such as arizona.edu for U of A and apple.com for Apple Computer, it is divided numberwise into networks and subnets, such as 128.196.0.0 or 130.43.0.0. for U of A 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 128.196.0.0 network physically resides at the University of Arizona. However, if a machine belonging to Apple were to be plugged into the UofA network, its name would still be something.apple.com even though its number would be 128.196.xxx.yyy. In this case, however, Apple and the U of A would share nameservice responsibility for this system: Apple for the name-to-number nameservice, and UofA for the number-to-name nameservice. The primary job that the domain name system 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 prep.ai.mit.edu. Also, it must map from IP addresses back to names, for example in order to provide some level of authentication as with the r commands.

The 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 128.196.120.82 is given as 82.120.196.128.IN-ADDR.ARPA