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This chapter is from the book

This chapter is from the book

Introduction to DNS

You probably use DNS every day, whether or not you are familiar with the underlying mechanism. Domain names are easy to use and remember. The ease with which you can access a Web site using domain names (such as http://www.microsoft.com or http://www.quepublishing.com) is a built-in simplicity that comes with a price: The DNS namespace is complex. DNS names are created as part of a hierarchical database that functions much like the directories in a file system. Hierarchies are powerful database structures because they can store tremendous amounts of data while making it easy to search for specific bits of information. Before examining the specifics of the DNS namespace hierarchy, let's review some rules about hierarchies in general.


Before getting into the details of a hierarchy, we should review some terms:

  • Tree—This is a type of data structure with each element attached to one or more elements directly beneath it. In the case of DNS, this structure is often called an inverted tree because it is generally drawn with the root at the top of the tree.

  • Top-level domain (TLD)—TLD refers to the suffix attached to Internet domain names. There is a limited number of predefined suffixes, and each one represents a top-level domain. The more popular TLDs include .COM, .EDU, .GOV, .MIL, .NET, and .ORG.

  • Node—A node is a point at which two or more lines in the tree intersect. In the case of DNS, a node can represent a TLD, a subdomain, or an actual network node (host).

  • Fully qualified domain name (FQDN)—A domain name that includes all domains between the host and the root of DNS is an FQDN. For example, http://www.microsoft.com is an FQDN.

  • Leaf—A leaf is an item at the bottom of a hierarchical tree structure, and it does not contain any other objects.

  • Zone—A DNS zone is a logical grouping of hostnames within DNS. For example, quepublishing.com is considered the forward lookup zone for Que Publishing. It is the place where the information about the Que Publishing hosts is contained within DNS.

In DNS, containers called domains hold the information. The hierarchy starts with a root container, called the root domain. The root domain doesn't have a name, so it is typically represented by a single period, as shown in Figure 3.1. The root domain contains pointers to all TLDs, which are directly below the root domain. These domains are also sometimes called first-level domains. Lower-level domains are second-level, third-level, and so on. Every domain name has a suffix that indicates which TLD domain it belongs to. There is only a limited number of such domains as defined by Request for Comment (RFC) 1591. Following are some of the more common TLDs:

Figure 3.1Figure 3.1 This portion of the DNS hierarchy shows the location of two domains in the DNS database in relation to the rest of the DNS database.

  • .COM—Intended for commercial entities, but it has become the overwhelming favorite top-level domain (example of .COM: area51partners.com)

  • .EDU—Intended for higher-education institutions, such as four-year colleges and universities (example of .EDU: berkeley.edu)

  • .GOV—Intended for use by agencies of the U.S. Federal Government (example of .GOV: whitehouse.gov)

  • .MIL—Intended for use by agencies of the U.S. military (example of .MIL: af.mil)

  • .NET—Intended for use by network providers and organizations dedicated to the Internet, such as Internet service providers (example of .NET: ibm.net)

  • .ORG—Intended for nonprofit or noncommercial establishments, such as professional groups, charities, and other such organizations (example of .ORG: npr.org)


DNS country codes Two-letter country code TLDs also exist for nearly all countries on the planet. Examples include .US for the United States, .CA for Canada, .JP for Japan, and .UK for the United Kingdom. New TLDs are constantly being added to meet the requirements for new domain names on the Internet. Recent additions include .BIZ and .INFO, among others.

Fully Qualified Domain Names (FQDNs)

As we have discussed, DNS is used to translate a hostname to an IP address. The FQDN name typically looks something like the following:


This is known as the host's fully qualified domain name (FQDN) because it lists the host's precise location in the DNS hierarchy. The DNS name in the example represents the host FILESVR042 in the subdomain CORPORATE (this is frequently a department or division in a company), which is in the subdomain MCSEWORLD (this is frequently the name of the company or organization that has registered the domain), which is in the TLD .COM.


Fully qualified domain names Make sure you have a good understanding of what an FQDN is and how it is represented.

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