What Happens When You Type a URL Into Your Browser? (Interview Question Guide)

What Happens When You Type a URL Into Your Browser? (Interview Question Guide)

posted 4 min read

One of the most common technical interview questions is, “What happens when you type a URL into a browser and press Enter?

While it might seem simple on the surface, this question tests your understanding of networking, web technologies, and system interactions. Let’s break it down step by step

1. URL Parsing

When you type a URL (e.g., https://example.com/page) into your browser, the browser parses it into its components:

  • Protocol: Determines how to communicate (e.g., https or http).

  • Domain Name: Specifies the website’s address (e.g., example.com).

  • Path: Identifies the specific resource (e.g., /page).

  • Query Parameters: Provides additional data (optional, e.g., ?id=123).

The browser identifies whether the request requires a secure connection (HTTPS) or a standard connection (HTTP).

2. DNS Lookup

The browser contacts a DNS (Domain Name System) server to translate the domain name (example.com) into an IP address (e.g., 192.168.1.1). This step involves multiple layers:

  • Checking the browser’s cache for stored DNS records.

  • Querying the operating system’s cache.

If no match is found, querying a DNS resolver, which may contact other DNS servers until it finds the IP address.

CDN Involvement

For websites using a Content Delivery Network (CDN), the DNS lookup often resolves to the nearest CDN edge server instead of the origin server. This ensures resources are served from the closest location to the user, reducing latency.

3. Establishing a Connection (TCP or QUIC)

Once the IP address is resolved, the browser initiates a connection to the server.

HTTP/1.1 and HTTP/2

1.The browser sends a SYN packet to the server.

  1. The server responds with a SYN-ACK packet.

  2. The browser sends an ACK packet back to the server.

  3. This process simply ensures a reliable connection between the browser and the server.

HTTP/3

With HTTP/3, the connection uses QUIC, a transport protocol based on UDP. QUIC eliminates the need for the three-way handshake, significantly reducing connection setup time. This is especially beneficial for latency-sensitive applications or networks prone to packet loss.

4. SSL/TLS Handshake (if HTTPS)

For secure connections (HTTPS), an additional step occurs: the SSL/TLS handshake.

This process:

  • Authenticates the server’s identity using a digital certificate.

  • Negotiates encryption algorithms to secure the data.

  • Establishes an encrypted session for safe communication.

5. Sending the HTTP Request

With the connection established, the browser sends an HTTP/HTTPS request to the server or CDN edge server.. Key components of the request include:

  • Request Method: Defines the action (e.g., GET for retrieving data).

  • Headers: Provide additional information (e.g., User-Agent, cookies).

  • Body: Contains data (for methods like POST).

6. Server Processing

The server receives the request and processes it:

If it’s a static resource (e.g., an image or HTML file), the server retrieves it from storage.

For dynamic content, the server may run scripts, interact with a database, or perform other computations to generate the response.

7. Sending the HTTP Response

The server sends an HTTP response back to the browser. This includes:

  • Status Code: Indicates the request’s success (e.g., 200 OK, 404 Not Found).

  • Headers: Contain metadata (e.g., Content-Type, Cache-Control).

  • Body: Contains the actual content (e.g., HTML, CSS, JavaScript).

CDN Caching:

If the request is routed to a CDN:

Cache Hit: The CDN serves the resource directly.

Cache Miss: The CDN forwards the request to the origin server, fetches the resource, and caches it for future requests.

8. Browser Rendering

The browser processes the response and renders the web page. This involves:

  • HTML Parsing: Builds a DOM (Document Object Model) tree from the HTML.

  • CSS Parsing: Constructs a CSSOM (CSS Object Model) tree for styling.

  • JavaScript Execution: Executes scripts to manipulate the DOM and CSSOM.

  • Layout and Painting: Calculates element positions and paints pixels on the screen.

Additional resources like images, fonts, and videos are fetched through subsequent HTTP requests.

9. Caching and Optimization

To improve performance, the browser and CDNs cache resources for future use:

  • The browser stores frequently used files locally.
  • CDNs cache content at edge locations to serve users faster.
  • List item

Caching reduces the need for repeated network requests, improving load times during subsequent visits.

10. User Interaction

Once the page is displayed, the browser enables interaction (e.g., clicking, typing). Any further actions might trigger JavaScript or additional network requests (e.g., via AJAX or Fetch API).

How to Tackle This in an Interview

1. Tailor Your Answer to the Role

If applying for a frontend role, focus on rendering and optimization.

For backend roles, emphasize server processing and HTTP requests.

2. Be Concise, Yet Comprehensive

Avoid overwhelming the interviewer with excessive detail. Aim to convey a clear understanding of the key steps.

3. Use Analogies (If Needed)

Relate technical processes to real-world concepts to demonstrate your ability to simplify complexity.

By understanding and articulating this process, you not only answer the question but also showcase your grasp of fundamental web technologies, setting yourself apart in interviews. Happy preparing!

If you read this far, tweet to the author to show them you care. Tweet a Thanks
This is a fantastic breakdown of such a seemingly simple but deceptively deep question! I appreciate how you've clearly explained each step in a way that's easy to follow while maintaining technical accuracy. The 'How to Tackle This in an Interview' section is particularly helpful, especially the advice on tailoring the answer to the role and using analogies.

One suggestion: It might be helpful to include a brief mention of modern advancements like HTTP/3 or CDN involvement in the process, as these are increasingly common in real-world scenarios. Overall, great job on such a detailed and useful article!
This is a concise explanation of something we literally do everyday.
Weldone, Khiilara
James, you're absolutely right ...that does make sense.

I adjusted the article to reflect those as well. Thank you !
That's it. Thank you.

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