What is DNS and the DNS Hierarchy

Posted on August 22nd, 2016

The DNS is an integral part of the Internet as it would not exist without it. The DNS is in a hierarchy in which the members of it are ranked according to the relative status. We are going to see the DNS hierarchy in detail here.


What is DNS?

The DNS stands for Domain Name System. The prominent intention of DNS is to translate domain names to the IP addresses. Even though there are there are domain names for all the websites, there are IP addresses also for them.The Internet uses this IP addresses to identify the websites. The IP address is a numerical data incorporated with four parts separated by dots(.). This numerical value is not easy to remember, so domain names were created, which are easily memorable. The DNS is responsible for translating these domain names to the IP addresses.

The DNS is a worldwide network that collectively forms a database of domain names and IP addresses. This database is a global one. The hierarchy consists of DNS servers. A DNS server can be defined as the following.


DNS Server

A DNS server is also a web server. Its primary objective is to interact with the aforementioned database. These DNS servers translate the domain name entered into the URL area of a web browser to the corresponding IP address. There are thousands of DNS servers worldwide which form the Domain Name System which currently is the largest digital database.


Fully Qualified Domain Name (FQDN)

It is essential to know about Fully Qualified Domain Name (FQDN), to understand the DNS hierarchy. A FQDN is the domain name that specifies its exact location in the DNS hierarchy. It specifies all domain levels including the top-level domain and the root zone. It consists of two parts, the host name and the domain name. An example of FQDN in a mail server is “mail.mydomain.com” where “mail” is the host name and the “mydomain.com” is the domain name. A fully qualified domain name is supposed to have little ambiguity. FQDN is otherwise called an absolute domain name.


DNS Hierarchy

The DNS hierarchy is comprised of the following elements:

1) Root Level

2) Top Level Domains

3) Second Level Domains

4) Sub-Domain

5) Host


DNS Root Zone

The DNS root zone is the highest level in the DNS hierarchy tree. The root name server is the name server for the root zone. It answers the requests for records in the root zone and answers other requests by providing a list of authoritative name servers for the appropriate TLD (top-level domain). The root nameservers are very important because they are the first step in resolving a domain name. These are the authoritative nameservers which serve the DNS root zone. These servers contain the global list of the top-level domains. The root zone contains the following:

1) Organizational hierarchy such as .com, .net, .org.

2) Geographic hierarchy such as .uk, .fr, .pe.


The root DNS servers are operated by 12 different organizations.

1) Verisign

2) University of Southern California

3) Cogent

4) University of Maryland

5) NASA AMES Research Center

6) Internet Systems Consortium

7) US Department of Defense

8) US Army Research Lab

9) Netnod

10) RIPE


12) WIDE


 Top Level Domains

The next level in the DNS hierarchy is Top level domains. There are many TLDs available at the moment. As we have seen the TLDs are classified as two sub categories. They are organizational hierarchy and geographic hierarchy. Let us see each in detail.


Organizational Hierarchy

Domain                                    Purpose

com                                         Commercial organizations

edu                                          Educational institutions

gov                                           Government institutions

mil                                           Military groups

net                                           Major network support centers

org                                           Nonprofit organizations and others

int                                            International organizations



Geographic hierarchy

In the geographic hierarchy, each country is assigned with two letter codes. These codes are used to identify countries.

For example, take the domain name images.google.com

Here, the “.com” is the top-level domain. It is called as tld in short. This is the next component in the DNS hierarchy. A TLD can have many domains under it. For example, a .com tld can have linux.com, centos.com, ubuntu.com, etc.

Sometimes, there is a second level hierarchy to a tld. They deal with the type of entity intended to register an SLD under it. For example, for the .uk tld, a college or other academic institution would register under the .ac.uk ccSLD, while companies would register under .co.uk.


Second Level Domains

The next level in the DNS hierarchy is the Second Level Domains. This is the domain that is directly below the tld. This is the main part of the domain name. It can vary according to the buyer. There are no limits here as the tlds. Once the domain is available anyone can purchase it. If the domain is unavailable at the moment, same 2nd level name with other tlds is the best option.



The sub-domain is the next level in the DNS hierarchy. The sub-domain can be defined as the domain that is a part of the main domain. The only domain that is not also a sub-domain is the root domain. Suppose two domains. one.example.com and two.example.com. Here, both the domains are the sub-domains of the main domain example.com and the example.com is also a subdomain of the com top level domain.

This is the DNS hierarchy and elements of the DNS hierarchy.


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