Domain Name System - Part 1

This article explains about Domain Name System which is popularly called as DNS in computer and internet world. I have covered and consolidated few definitions of DNS which are available in internet.  Even though few of these contents are taken from various sources of internet, you may find uniqueness in order of explanations and examples are being used in my post.  Let us jump into DNS ocean now.
The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, services, or other resources connected to the Internet or a private network. Most prominently, it translates more readily memorized domain names (Example: www.example.com) to the numerical IP addresses (Example: 182.663.43.44) needed for locating and identifying computer services and devices with the underlying network protocols of internet.  In the Internet protocol stack, DNS comes in Application layer as below.


DNS will resolve names to numbers. that means, it will resolve domain names (www.yahoo.com) to IP address (74.125.44.25).  Please have a look at below picture.



DNS is like a phone book.  In phone book, when you look at any number, you will look at name first and find number.  Same way when you search for yahoo.com, your browser will search IP address of that server with help of domain name yahoo.com. There are plenty of technologies involved when you start typing any domain name in browser until you IP address of it.  let me explain all the sequence of steps below.

Below is Bird’s Eye View of DNS requests:

• Generally every layer of DSN requests, group of servers are responsible and answerable for single DNS request as below.

DNS Look up in Depth:

• When you start typing yahoo.com in your browser, firstly your browser will look at cache of local system to get IP address of yahoo host. If local cache doesn’t have IP then your browser will contact Resolver host which is located at ISP (Internet service provider) of your home internet.  If Resolver doesn’t have any answer then contact Root Servers, which are set of servers maintained by 13 different organizations.  There are approximately 380 root servers and divided into 13 groups.

• Here Root server is not going to know what is IP address for Yahoo.com; Instead, it does know how to redirect resolver to find IP address of yahoo.com host.  Root server will send TLD (Top level domain) server IP details to resolver of ISP as below. 

Basically, Root server will share 4 important piece of information as below.

1. yahoo.com: the domain to to look up. (www.yahoo.com)
2. IN: the DNS Class (IN: Internet)
3. NS or A: the record type (NS: Name server Address / A: Address)
4. TTL (Time to Live).  Generally, it is valid for 2 days.

Below is example of Root server response.

• Then, resolver will contact TLD (Top level domain) servers to get IP address of Top level domains (Example: .COM, .NET, .ORG, and etc.,) as below.  This particular TLD server manages .COM domains.

• Then, .COM’s TLD server will redirect ISP resolver server to Authoritative Name Server (NS54.WorldNIC.com).

Basically, TLD server will share 4 important piece of information as below.

1. yahoo.com: the domain to to look up. (www.yahoo.com)
2. IN: the DNS Class (IN: Internet)
3. NS or A: the record type (NS: Name server Address / A: IPv4 Address / AAAA: IPv6 Address)
4. TTL (Time to Live).  Generally, it is valid for 2 days.

Below is example of TLD server response.

• Then, Resolver will ask Authoritative Name Server (NS54.WorldNIC.com) IP address for yahoo.com server.  Authoritative name server knowing everything about domain, including IP address.

• then, Authoritative will Resolver will ask Authoritative Name Server (NS54.WorldNIC.com) IP address for yahoo.com server.  Authoritative name server knowing everything about domain, including IP address.  Authoritative name server will give IP address to Resolver.

Basically, Authoritative server will share 4 important piece of information as below.

1. yahoo.com: the domain to to look up. (www.yahoo.com)
2. IN: the DNS Class (IN: Internet)
3. NS or A: the record type (NS: Name server Address / A: IPv4 Address / AAAA: IPv6 Address)
4. TTL (Time to Live).  Generally, it is valid for 2 days.

Below is example of Authoritative server response.

• Lastly, Resolver will send IP address to your browser, and stores IP address in cache of ISP’s Resolver host.  This cached data will be used in case if it receives other query for yahoo.com.

DNS Look up in methodology:
Generally, DNS search process either Iterative or Recursive.  But preferred DNS search process it mix of these methods in real world scenarios.


Below is mixed mode of DNS record search process.


I hope this article may help you to understand about DNS look up process and terminology is being used in IT world.  I tried to give this article more simplest form which will help people to look basic facts of DNS looks up and step by step process of it.  Further, i am planning to prepare article to explain about Domain name registration process and related terminologies.

Thank you all…I will meet you in my next post…

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