Chapter 7

How the Domain Name System (DNS) Works


The heart of intranets and the Internet is the Domain Name System (DNS), the way in which computers can contact each other and do things such as exchange electronic mail, or display Web pages. The Internet Protocol (IP) uses Internet address information and the DNS to deliver mail and other information from computer to computer.

You may not realize that every IP address on the Internet is actually a series of four numbers separated by periods (called dots), such as It would be impossible for you to remember these numeric addresses when you wanted to send e-mail or visit a site. Also, because sometimes numeric IP addresses change, you would never be able to know every time those numeric addresses change. The DNS solves these problems.

The DNS creates a hierarchy of domains or groups of computers and it establishes a domain name (also known as an Internet address) for each computer on an intranet or the Internet, using easily recognizable letters and words instead of numbers. Major domains also have the responsibility for maintaining lists and addresses of the domains that are underneath them. That next level of domains is responsible for the following level down and so on.

An Internet address is made up of two major parts separated by an @ (pronounced at) sign. The first part of the address-to the left of the @ sign-is the user name, which usually refers to the person who holds the Internet account, and is often that person's login name. The second part of the address, to the right of the @ sign, is the host name or domain name, which identifies the specific computer where the person has an Internet mail account. Often, the domain name will be the name of the intranet.

The rightmost portion of the domain section of the address identifies the largest domain and kind of organization where the person has his or her address. Common domains in the United States are com for commercial; edu for education; gov for government; mil for military; net for network (companies and groups concerned with the organization of the Internet); and org for organization. Outside the United States, only two letters are used to identify the domains, such as au for Australia; ca for Canada; uk for United Kingdom; and fr for France.

Typically, an intranet will have its own domain, often the name of the company that owns it. Mail to be delivered begins with a request to an intranet nameserver first. If the host receiving the mail is on the intranet, the nameserver will be able to translate the Internet address into the numeric IP address, and so the mail can be delivered. If the host isn't on the intranet, the nameserver may have to contact an Internet nameserver. It does this by contacting an Internet root domain name server, which then tells it which Internet name server to contact. That Internet nameserver will be able to translate the Internet address into the numeric IP address, and again, the mail will be able to be delivered.

How Intranet Domain Name System (DNS) Servers Work

  1. When a particular URL needs to be contacted, the address with the URL must be matched to the true IP address. Your Web browser first goes to a local DNS server on the company intranet to get this information. If the IP address is a local one, the DNS server will be able to resolve the URL with the IP address. It will send the true IP address to your computer.
  2. Your Web browser now has the true IP address of the place you're trying to locate. It uses that IP address and contacts the site. The site sends you the information you've requested.
  3. If the information you have requested isn't on your intranet-and if your local DNS server doesn't have the IP address-the intranet DNS server must get the information from a DNS server on the Internet. The intranet DNS server contacts what's called the root domain server, which is maintained by a group called the InterNIC. The root domain server tells the intranet server which primary nameserver and secondary nameserver has the information about the requested URL.
  4. The intranet DNS server now contacts the primary nameserver. If the information cannot be found in the primary nameserver, the intranet DNS server contacts the secondary server. One of those nameservers will have the proper information. It will then pass the information back to the intranet DNS server.
  5. The intranet DNS server sends the information back to you. Your Web browser now uses the IP address to contact the proper site.

When someone on an intranet wants to contact a location-for example, to visit a Web site-they will type in an address, such as In fact, though, the Internet doesn't truly use these alphanumeric addresses. Instead, it uses IP addresses, which are numerical addresses, in four 8-bit numbers separated by dots, such as A DNS server, also called a nameserver, matches alphanumeric addresses to their IP addresses, and allows you to contact the proper location.