Domain Name Server (DNS) is analogous to a phone book where people can look up a person name through numerical phone number. A DNS server is a database that host a database of public IP addresses (e.g. 64.233.160.0) and their associated hostname (e.g. google.com).
Whenever we type domain name in the address bar of our browser (e.g. google.com), that domain will be delivered to DNS server. Inside DNS server the URL will be mapped with its corresponding IP address making it possible for the web request to reach the the target server.
