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What Happens When You Type a URL in the Browser?

When you enter a URL in the browser and press Enter, multiple processes occur to fetch and render the webpage. Here's a step-by-step breakdown:

1. URL Processing​

  • The browser checks the URL for validity.
  • If no protocol (http:// or https://) is specified, it defaults to one.

2. DNS Resolution​

  • The browser looks up the domain's IP address from its cache.
  • If not found, it queries the OS's DNS resolver.
  • If still unresolved, a DNS server (like Google’s 8.8.8.8) is used to resolve the domain to an IP address.

3. Establishing a Connection​

  • A TCP handshake occurs: SYN → SYN-ACK → ACK.
  • If HTTPS is used, a TLS/SSL handshake is performed for secure communication.

4. Sending HTTP Request​

  • The browser sends an HTTP request containing:
    • Request Method (GET, POST, etc.)
    • Headers (User-Agent, Cookies, Authorization, etc.)
    • Optional body data (for POST, PUT requests)

5. Server Processing​

  • The web server (e.g., Nginx, Apache) receives the request.
  • It may interact with a database or perform backend operations.
  • The server generates an HTTP response (HTML, JSON, etc.).

6. Receiving HTTP Response​

  • The server responds with:
    • Status Code (200 OK, 301 Redirect, 404 Not Found, etc.)
    • Headers (Content-Type, Cache-Control, etc.)
    • Response Body (HTML, CSS, JavaScript, JSON, etc.)

7. Rendering the Page​

  • The browser parses the HTML document.
  • Additional requests are made for linked resources (CSS, JS, images, etc.).
  • CSS is applied, JavaScript is executed, and the DOM is built.
  • The browser progressively renders the page.

8. Handling JavaScript & Additional Requests​

  • JavaScript may trigger further requests (AJAX, WebSockets, etc.).
  • The page may update dynamically without a full reload.

9. Caching & Performance Optimization​

  • The browser caches static assets (CSS, images, scripts) for future use.
  • CDNs optimize resource delivery.
  • Compression (e.g., Gzip, Brotli) reduces payload size.
  • Lazy loading and prefetching improve performance.

10. User Interaction​

  • The user interacts with the page (clicks, scrolls, inputs).
  • JavaScript handles events dynamically.
  • Single Page Applications (SPAs) may update content without full page reloads.

This is a high-level overview. Each step includes optimizations, security checks (DNS caching, HSTS, CSP), and browser-specific handling. 🚀