Q87- What happens when you type a URL in the browser and press enter?

Typing a URL in the browser triggers a complex chain of events involving the browser, network, and server. Here’s a step-by-step breakdown:

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1️⃣ Browser Checks Cache

• The browser checks if the page is already cached (HTML, CSS, JS, images).
• If cached and valid → serves it without a network request.
• Otherwise → proceeds to network request.

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2️⃣ DNS Lookup

• The browser resolves the domain name (e.g., example.com) to an IP address using DNS.
• Checks local cache → OS cache → router → ISP DNS servers.

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3️⃣ TCP Connection

• Browser initiates a TCP handshake with the server (IP from DNS).
• Usually uses port 80 for HTTP or 443 for HTTPS.

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4️⃣ TLS/SSL Handshake (for HTTPS)

• Establishes a secure encrypted connection between browser and server.
• Certificates are verified to ensure authenticity.

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5️⃣ HTTP Request

• Browser sends an HTTP request to the server:
  • Method: GET
  • Headers: User-Agent, Accept, Cookies etc.
  • URL path, query params

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6️⃣ Server Processing

• Server receives request, processes it, and generates a response:
  • Could be static HTML or dynamically generated content (via Node, PHP, Python, etc.)
  • Includes status code (200, 404, 500, etc.)
  • Response headers (Content-Type, Cache-Control, etc.)

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7️⃣ Response Sent to Browser

• Browser receives HTML first
• Starts parsing HTML and discovers CSS, JS, images → sends additional requests (HTTP/2 can handle multiple in parallel)

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8️⃣ Browser Rendering

1. HTML Parsing → DOM tree
2. CSS Parsing → CSSOM tree
3. JS Execution → may modify DOM or CSSOM
4. Render Tree → layout calculation → painting on screen
5. Repaints/Reflows as needed (if JS modifies layout)

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9️⃣ Final Page Load

• Event listeners (DOMContentLoaded, load) are triggered.
• Page becomes interactive.
• Browser may continue fetching deferred resources like images, videos, or API calls.

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⚡ In short:

Typing a URL starts from DNS resolution, establishes a TCP/HTTPS connection, sends an HTTP request, gets a response, and the browser parses HTML, CSS, and JS to render the page.

It’s a complex pipeline involving caching, networking, security, and rendering.