DNS (Domain Name System / server / service) is an internet service that translates domain names into IP addresses.
Why We Need DNS?
Domain names are alphabetic. It is easier for human to read and remember, however web servers only recognize IP addresses and they need IP address to connect other hosts. Therefore, we need DNS to resolve human-readable domain into computer-readable IP addresses.
By typing the following IP address to the web browser, you will end up reaching the same website. Same result will be achieved in typing the typical URL of the website in the web browser.
184.108.40.206 = google.com
220.127.116.11 = facebook.com
Why are there several nameservers for my domain?
Additional DNS act as backup in case one DNS server fails to translate the domain name. It will ask another one, and so on. When DNS is down, it will hinder the domain to be seen by internet users.
google.com have 4 Name servers
Name Server: ns3.google.com
Name Server: ns1.google.com
Name Server: ns2.google.com
Name Server: ns4.google.com
facebook.com have 2 Name servers
Name Server: a.ns.facebook.com
Name Server: b.ns.facebook.com