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Anyone who uses the Internet has used DNS, even if they don't realize it. The simplest explanation for DNS is this: it converts human readable names into machine readable IP addresses. DNS is similar in theory to a phone book, where names which are relatively easy to remember are mapped to numbers which are difficult to remember. DNS was first implemented in the earliest days of the Internet, when the number of systems connected to the network became high enough that it became difficult to track every systems address manually. DNS was conceived as an automated system to do that job. In practice, DNS is very different from a phone book; DNS is a hierarchical system, which uses a number of servers to service each request. Unless a request is cached, but we will cover that later. The layout of the hierarchical system is covered in the later lesson, the DNS name space, and the lesson following that entitled the DNS request process, explains how that hierarchy is used to resolve a name to an address. The primary use for DNS is to resolve a name to an IP address, this is known as a forward look up. DNS can also be used, if you know an IP address and want to find the name it is associated with. This process of resolving IP address to a name is called reverse look up. DNS uses clients and servers; while a DNS server is a well-defined entity, DNS clients come in a few different forms. The two main types of DNS clients are integrated clients, and command line clients; both are present on almost every system including windows, UNIX and others. An integrated client, is built into an application such as Internet explorer; the integrated client usually uses a DNS client service provided by the operating system to resolve names to addresses for use within the application. When you type www.Microsoft.com, it resolves that name to an address, and brings up the web site associated with that address in Internet explorer. Integrated clients can exist in all sorts of applications such as web browsers, email applications, and even server applications such as email or database servers usually have integrated DNS clients. A command line DNS client, like NS look up shown here, is used to manually map a name to an IP address. These clients are typically used for trouble shooting DNS problems, and aren't much use for a typical end user. The command-line clients in NS look up and dig, are covered later in the course.