DNS – Domain Name Server
In a nutshell: Like your phone book, a place where you find phone number.
Let’s say you enter www.google.com into the browser’s address bar. The first thing the browser would need to do is find the IP address associated with that particular domain name, so that it can connect to Google’s server and fetch the page. There are a few different ways for a browser to translate a domain name into an IP address(for now, refer to IP address as the server unique identifier, like a unique phone number per person). First, it would look up its own cache to see if it already knows the IP address associated with this domain. This would usually occur if you have visited that website in the past few minutes. If nothing is found, the browser keeps looking and asks your computer’s operating system whether it has the IP address of the website in question. Still, if nothing is found, the request goes to your Internet provider’s DNS (Domain Name Server).
The DNS belonging to your ISP(Internet Service Provider) will generally be able to find the needed IP address and send it back to your browser, as it keeps the IP addresses of commonly visited websites in its cache. If it can’t find the record, it will start what is called a recursive search. It first starts at the root name server, then connects to the .com name server and finally reaches the google.com name server, which forwards the IP address of www.google.com to your ISP’s DNS. The DNS then sends the IP address to your browser. The process of accessing a website can be compared to wanting to call someone without having their phone number. You would first check to see if you can remember that person’s phone number in your head. If you don’t, you would check inside your phone’s contacts. If you still don’t have the number, you would then look up the number in the phone book.