Key component of DNS Domain Name space FQDN

root domain "." as the top.
Below the root domain the top level or first level domain
is .com .edu .org .us. au. .in .co.in .info .net
Second level domain is registered by individual or organization.
Such us Microsoft.com,computergraphics.com, database.com,learning.com interview.com Question.org,tips.org Second level domain has the many sup domains just like for interview.com it has the sub domain support.interview.com, question.interview.com, administrator.interview.com,
And there sub domain have any host.Fully qualified domain name  Domain name space Key component of DNS
Like computer1.question.interview.com, server1.adminsitrator.interview.com,

For fully qualified domain it has the every level what is FQDN

FQDN Fully Qualified Domain name
Like
first level domain .com
second level domain interview.com
sub domain administrator .interview.com
final one host name of domain server1.administrator.interview.com
This is called fully qualified domain name.
server1.administrator.interview.com

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