Whenever we use internet, we have to enter the name of website in URL 
address bar. These names and address URL of website is a domain names. 
It is formed by the rules and procedures of domain name system. In other
 words any name register under Domain name services is a domain name. These domain names are register under the Registrar companies like BigRock, godaddy etc.
What is need of domain names?
Yes whole web world is totally depend on domain names because without 
domain name it not possible to serve any site. Without domain we will 
not able to open any page or file. Suppose without domain like 
Google.com, Facebook.com etc how we can open it. ? If there are no 
defined names of web pages how server and web Protocols open that 
particular Page or website. Many Times we find DNS error occur on
 our internet. This error may be because of domain name is not  parked 
on particular web page or many other reasons.
Domain name is a particular name which point out that particular site for which that domain name is used. Every web 
service need domain names while it is sub-Domain or Top level 
Domain (TLDs)  which includes and categorizes under  Generic Top-level 
domain (gTLDs) such as .Com, .Net, .org, .info etc. and  Country code 
Top-level Domain (ccTLDs) such as .in, .us, .it, .pk are some 
extensions.
For example Google.com, Facebook.com, YouTube.com are 
some most popular Top-level domain names.
We can also make sub-domain name of any Top-level domain name. while 
these sub-domain also use for many purpose of creating different web 
page from web page of TLD site. mostly every TLD domain use www as www.sitename.com
While if we use TLD as sub-domain is like gmail.google.com which is 
sub-domain of Google.com. Every domain name can make many different 
sub-domains. 
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