Revisiting the Domain Name System (DNS), Part 2

Anyone who uses the Internet to send an E-mail or browse the Web uses the Domain Name System (DNS) without even realizing it. DNS is an incredibly important, but completely hidden, part of the Internet. The DNS forms one of the largest and most active distributed databases on the planet. Without DNS, the Internet would grind to a halt very quickly.

When you use the Web or send an E-mail message, you use a domain name to do it. For example, the URL contains the domain name So does the e-mail address sales@ Human-readable names like, though easy for people to remember, do not provide the necessary IP address information the machines use to communicate with each other.

The DNS allows you to connect to another networked computer or remote service by using its user-friendly domain name rather than its numerical IP address. Every time you use a domain name, you use the Internet’s DNS to translate the human-readable domain name into the machine-readable IP address. During a day of browsing and e-mailing, you might access these servers hundreds of times!

As a CCNA or CCNP, it is vital that you be able to install, configure, maintain, and troubleshoot the various operational areas of the DNS system, both locally and, possibly, on a world-wide level. In this post, we’ll take a look at the DNS in more detail so you can understand how it works and appreciate its amazing capabilities.

As we have been discussing, DNS translates domain names to IP addresses. This process sounds like a relatively simple task. And it would be, except for five factors:

  1. There are currently billions of IP addresses in use. And, most machines have a human-readable name as well.
  2. There are many billions of DNS requests made every day. A single user can easily make a hundred or more DNS requests a day. Compounding that fact, there are hundreds of millions of people and machines using the Internet daily.
  3. Domain names and IP addresses change daily.
  4. New domain names get created daily.
  5. Millions of people do the work to change and add domain names and IP addresses every day.

The DNS is basically a database, and no other database on the planet gets this many requests. Additionally, no other functional database currently in use has millions of people changing it every day. These factors are what make the DNS so unique.

There are some additional factors that impact the DNS process that must be considered.

Remember that every device that has a presence on the Internet must have its own unique IP address. Some of the basic rules of IP address assignment dictate that a server usually has a static IP address that does not change very often. On the other hand, a home machine that is connecting through a modem often has an IP address that is dynamically assigned by the Internet Service Provider (ISP) when you log in. That IP address is unique for your session and may be different the next time you log in. In this way, an ISP only needs one IP address for each modem it supports, rather than for every customer.

It should be noted that as far as the machines on the Internet are concerned, an IP address is all that you need to connect to a server. For example, you can type in your browser and you will arrive at a machine that contains a Web site such as Bicycleheaven.Com. Domain names are strictly a human convenience.

To serve requests for resolution, DNS uses the User Datagram Protocol (UDP) header at Layer 4 of the OSI model. In addition, the DNS functions on port number 53 of the OSI model. DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server. The Transmission Control Protocol (TCP) is used when the response data size exceeds 512 bytes or for tasks such as inter-zone transfers.

In addition, some operating systems, such as HP-UX, are known to have resolver implementations that use TCP for all queries, even when UDP would suffice.

In my next post, we will examine some of the more technical aspects of the DNS process.

Author: David Stahl

In this article

Join the Conversation