WebJunction

DNS:a simple concept

If you have ever used a phone book, then you are already familiar with the idea behind DNS. You probably have dozens and dozens of names in your memory - family, friends co-workers, etc. But chances are you only have a few of their phone numbers committed to memory. The reason for this is simple: Names are much easier to remember than random seven- or ten-digit numbers. When the time comes to contact that high school friend or long-lost relative, you turn to a phone book. You have the name, but without the number you cannot make the call. The phone book has both.

DNS works in a very similar way. The Domain Name System associates an easy to remember domain name (for example, yahoo.com) with its complex numeric corresponding IP address (64.236.16.20).

Nearly every person has a unique phone number. Similarly, every website has a unique Internet Protocol address. (Internet Protocol is a set of standards for computer-to-computer Internet communication)

www.yahoo.com uses IP address 66.218.71.80

www.cnn.com uses IP address 64.236.16.20

www.dictionary.com uses IP address 66.161.12.119

Just as you cannot call a friend until you know his phone number, computers cannot exchange information on the Internet without knowing each other's IP address

DNS Servers

A DNS server is a computer that contains a database of domain names and their IP addresses. Based on the analogy above, the DNS Server is the phone book. If a DNS server cannot fulfill you request it will refer the request to another DNS server - and the request is passed along until the domain-name-to-IP-address match is made. There are many thousands of DNS servers operating on the Internet.

Part of the Internet configuration for any computer is the DNS server entry. This entry specifies which DNS servers that computer should use to match up IP addresses with domain names. This server is generally owned and maintained by your Internet Service Provider (ISP).

An Example…

This example will illustrate step-by-step a DNS lookup. It has been simplified somewhat to fit the scope of this article.

1. A computer user needs to access information from the New York Times website. The user knows that the domain name for the New York Times is nytimes.com.

2. The user opens their browser and types www.nytimes.com.

3. The first priority of the computer's Internet software is to figure out what IP address www.nytimes.com uses.

4. The computer has been pre-configured to use the DNS server at the ISP so it sends a request out to that DNS server. The request says “I need to contact www.nytimes.com: can you tell me the IP address?”

5. The DNS server at the ISP examines the request. The server checks its DNS tables but does not find an entry for www.nytimes.com.

6. Since DNS is a distributed system and all DNS servers cooperate, the server sends the request to a more authoritative DNS server in search of www.nytimes.com.

7. The second server in the chain has an entry for www.nytimes.com. It sends a reply to the ISPs DNS server that says “I've got a match for www.nytimes.com! It can be found at IP address 64.94.185.200”

8. The ISPs DNS server then sends this answer back to your computer. Once your computer receives the IP address it makes the “call” to the New York Times web server at 64.94.185.200.