News Archives

An Introduction to DNS

PFW.com - 2002 The Domain Name System (DNS), a worldwide directory that maps host names to IP addresses, is one of the most important components of the Internet. Each time you use your web browser or send an e-mail message, you're depending on the Domain Name System to help you, or your message, reach the proper online destination.

In the early days of the Internet, each connected host stored a copy of the network addresses and host names of every other connected host in a 'hosts' file on its hard drive. Periodically, this file would be updated by a central administrator, and then every host would need to download and store a new copy. However, as the number of machines connected to the Internet grew, this method of keeping track of host names and addresses quickly became unworkable and a new solution was needed. Enter the Domain Name System.

The key difference between DNS and the old 'hosts' file method is that DNS is a distributed database-that is, it has no central storage point of information. Each DNS server around the world contains the information for the domains that it represents (its local domains), as well as pointers to help it locate information for domains represented by other DNS servers (remote domains), no matter where in the world they are located. A related benefit is that instead of having all host name-to-address mapping handled centrally, each DNS server is administered separately, meaning updates to host names or addresses can be made more efficiently.

Here is a brief example of how the Domain Name System helps your web browser find your favorite web site:

1. You enter the address 'www.greatsite.com' in your web browser's Address line.
   
   
2. Your PC contacts the DNS server listed in its TCP/IP network configuration and asks for the IP address of www.greatsite.com.
   
   
3. The DNS server checks to see if greatsite.com is a local domain name. If so, it returns the IP address of www.greatsite.com to you. However, if greatsite.com is a remote domain name, the DNS server sends a request to a '.com root zone server' (a server that keeps track of all domain names ending in .com) and asks for the address of a DNS server that represents greatsite.com.
   
   
4. The root zone server returns the address of a server to which greatsite.com is a local domain name, and your DNS server then contacts that remote DNS server to request the IP address of www.greatsite.com.
   
   
5. The DNS server for greatsite.com passes the address of www.greatsite.com to your DNS server, which in turn passes the address to your PC, and your web browser contacts the web site.

This background process is similar to what happens when you send an e-mail message, and in each case the DNS lookup usually takes place within a few seconds. Clearly, the Internet as we know it wouldn't be the same without the Domain Name System to help us locate and identify Internet addresses. In a future article, we will examine at the various types of DNS records (e.g., MX, A) that each Domain Name System server stores in its database.