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As stated in earlier lessons, DNS is used everywhere on the Internet, as well as in many private networks. The number of uses for DNS is nearly unlimited, as almost any network service can benefit from it in some way or another. DNS is extremely flexible by design; with its broad range of applications, let's look at some of the more common ones. Every page on the World Wide Web accessed by name not by IP address uses DNS; e-mail uses DNS to get your mail to its destination. Microsoft active directory uses DNS as an integral part of the service. Finally, there're a range of other services that use DNS, and I'll briefly touch on how that works later in the lesson. The World Wide Web uses DNS extensively; the web is the most visible use of DNS on the Internet though it may lag behind e-mail in popularity. Every time you access a Web site by name, such as www.vtc.com, DNS references a host record to resolve that name to an IP address. The Web is actually one of the more simple uses for DNS as once the name is resolved, the web browser retrieves the content from the web server using the address. You can see DNS at work every time you load a page in your browser. E-mail is reported as the most popular use of the Internet based on the total number of users; without DNS, e-mail wouldn't function like it does today. E-mail uses DNS for mail routing; mail routing is used to get an e-mail that you send from your mail server to the recipient's mail server. This is facilitated by the mail exchanger record in DNS. The mail server first inspects the domain in the e-mail address called the host portion of the address; every thing before the @ symbol in an e-mail address is referred to as the user portion. While everything following the @ symbol is called the host portion. It then uses DNS to resolve the mail exchanger record for this domain to an IP address. Finally it uses the SMTP protocol to send the message to the receiving server's IP address resolved in the last step. Unfortunately it's hard to show an example of this process, but it occurs every time an e-mail is sent. Microsoft active directory uses DNS as one of the core building blocks in its infrastructure. DNS is used with active directory to maintain database of services on that network; these services are listed in DNS using service records. Service records allow any client in an active directory environment to locate any service it needs such as a printer. This DNS integration removes the requirement of knowing which server hosts are given a resource. As was the case in versions in Microsoft windows before active directory was released. Instead a client can use resources without knowing anything about the underlying network and server layout. DNS is used for a variety of other applications as well; any time you reference a host by its DNS name, DNS is used. This occurs regardless of the service you're using. Some examples of services that use DNS are telnet and SSH for remote system access to UNIX servers, some database client utilities, groupware clients and back-up utilities. Typically, these applications will reference the target server based on its host record.