The Domain Name System (DNS) is a critically important part of TCP/IP internetworks, especially the modern Internet, because it allows hosts to be accessed using easily-remembered names rather than confusing numerical addresses. Two different primary types of devices are involved in the operation of DNS: DNS name servers that store information about domains, and DNS resolvers that query DNS servers to transform names into addresses, as well as performing other necessary functions.

DNS resolvers are employed by Internet users on a continual basis to translate DNS names into address, but under normal circumstances, they are always invoked indirectly. Each time a user types a DNS name into a program such as a World Wide Web browser or FTP client —or even uses it in one of the other utilities in this section, such as ping or traceroute—the resolver automatically performs the name resolution without the user having to ask. For this reason, there is no need for users to manually resolve DNS names into addresses.

However, administrators often do need to perform a DNS resolution manually. For example, when troubleshooting a problem, the administrator may know a host’s name but not its address; in the case of a security problem, the address may show up in a log file but the host name may not be known. In addition, even though users do not need to know the specifics of the resource records that define a DNS domain, administrators often need to be able to check these details, to make sure a domain is set up properly. Finally, admins also need some way to be able to diagnose problems with DNS servers themselves.