Subdomain

Subdomain is a domain name that is created under parent domain to identify a specific service, application, or section of a website.

For Example:

amazon.com
  |- login.amazon.com
  |- product.amazon.com
  |- support.amazon.com
  |- api.amazon.com
  |-mail.amazon.com

login .     amazon .      com
  |          |            |
subdomain domain  TLD(top level domain)

WHY SUBDOMAIN IS USED?

Companies use subdomain to seperate different services.

DNS Records

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DNS has different types of records. Comman records are

1. A Record- Maps hostname to an IPv4 address. For example 192.168.1.10
2. AAAA Record- Maps hostname to an IPv6 address. For example 2001:db0:1
3. CNAME Record- Maps one hostname to another hostname. For example

blog.amazon.com
       |
     CNAME
       |
amazon.blog.example-service.co

4. MX Record- Mail exchange record is used for email which specifies the mail servers responsible for recieving email for a domain .

5. TXT Record- Stores text information associated with domain, commonly for verification, and email security purpose.

6. NS Record- Name Server(NS) is a DNS server that stores and provides dns records for domain or DNS zone.

NOTE: DNS Zone is a managed portion of DNS records of a domain, where 
domain/subdomain's information.
 For Example- example.com
                  |- A
                  |- CNAME
                  |- MX
                  |- TXT
                  |- NS

Why NS?

Tells DNS where to find the offical DNS information for a domain.