What happens when you type a URL into a browser?

We’ve all done it—typed a URL into the address bar of a browser, hit “Enter,” and watched a website load in seconds. But have you ever wondered what happens behind the scenes? It might seem like magic, but it’s actually a complex yet fascinating process involving multiple systems working in sync to retrieve the web page you’re looking for. From translating domain names into IP addresses to establishing secure connections, every action is a crucial part of the browsing experience.

In this post, we will explore the various chains of events involved when you type a URL into your browser. This includes how your computer’s operating system, your router, and DNS servers across the internet all work together to finally render a webpage for you. Understanding this process not only helps identify where things can go wrong but also where performance issues may arise and how to ensure a fast, secure, and reliable experience for your users.

How it all works

1. Typing the URL

This is where the whole process begins. You type a URL (Uniform Resource Locator) like www.google.com into the web browser. The URL is a unique address of a webpage, and it includes several components:

Protocol: This tells your browser how to retrieve the webpage resources. Standard protocols include HTTP (Hypertext Transfer Protocol) and HTTPS (Hypertext Transfer Protocol Secure).
Domain name: This is simply the human-readable address of a website (e.g., google.com)
Path (optional): The path specifies which specific page or resource you want to access (e.g., /about to access the about page of a website or /contact to access the contact page of a website).

2. DNS Resolution – Translating the domain name to an IP address

After you hit enter, the first step is for the browser to translate the domain name into an IP address. This process is called a DNS (Domain Name System) resolution. A series of steps are involved in resolving the domain name to its corresponding IP address so that your web browser can retrieve the requested web page. Here’s a granular and detailed breakdown of what really happens during this process:

a. Browser Cache Check

Step 1: The first thing that the browser checks is its own cache to see if it has recently resolved www.google.com and stored the corresponding IP address.

Outcome: If the IP address is found in the cache and hasn’t expired, the DNS resolution process is complete, and the browser proceeds to initiate an HTTP or HTTPS connection to that IP address.

b. Operating System (OS) Cache Check

Step 2: If the IP address is not in the browser cache, the browser asks the operating system (OS) to resolve the domain name. The OS has its own cache (known as the DNS Resolver Cache) where it may store recently resolved domain names.

Outcome: If the IP address is found in the OS cache, the OS returns it to the browser, and the DNS resolution process ends here.

c. Router Cache Check

Step 3: If the OS cache does not contain the IP address, the request is forwarded to your local router (if you are home, it would simply be your router at home). The router also has a cache where it stores DNS resolutions.

Outcome: If the IP address is found in the router’s cache, it is returned to the OS, which then passes it back to the browser, and the DNS resolution is complete.

d. ISP DNS Resolver Query

Step 4: If the IP address is not found in any local caches (the browser, OS, or router), the request is sent to the DNS resolver provided by your Internet Service Provider (ISP). This resolver typically handles DNS requests for large number of users.

Outcome: If the ISP DNS resolver has the IP address cached, it returns it to your router, which then sends it back to your OS and browser.

e. Recursive DNS Query (if not cached)

Step 5: If the ISP’s DNS resolver does not have the IP address cached, it initiates a recursive DNS query to resolve the domain name. This involves querying multiple DNS servers in a hierarchical manner.

Step 6: The DNS resolver starts by querying one of the Root DNS servers. These servers do not know the IP address of www.google.com but can direct the resolver to the appropriate Top-Level Domain (TLD) DNS Server.

Step 7: The TLD DNS Server (in this case, one that handles .com domains) is queried next. It does not have the IP address for www.google.com either, but it knows the Authoritative DNS Server for the google.com domain.

Step 8: The DNS resolver then queries the Authoritative DNS Server for google.com. This server has the final authoritative answer and returns the IP address associated with www.google.com.

Outcome: The IP address is returned by the Authoritative DNS Server to the ISP’s DNS resolver.

More on Recursive DNS Query here -> Some DNS Topic

f. DNS Resolver Cache Update

Step 9: The ISP’s DNS resolver caches the IP address for www.google.com for future queries and then returns it to your router, which passes it to your OS and finally to your browser.