Narendra | SRE

Posted on May 23

What Really Happens When You Type a URL in Your Browser?

#networking #cloud #dns #linux

You type a URL. Press Enter. A beautiful website appears in milliseconds. But behind that single action, an entire chain of networking, security, and system-level operations takes place.

If you're learning DevOps, SRE, Backend Engineering, Networking, or System Design - this is one of the most important flows to understand.

🌍 Example

Suppose you type:

🌍https://google.com

what happens internally?
Let's break it down step-by-step.

⚡ High-Level Flow

Enter URL
↓
Browser Cache Check
↓
DNS Resolution
↓
TCP Handshake
↓
TLS/SSL Handshake
↓
HTTP Request Sent
↓
Load Balancer / Server
↓
Application Processing
↓
HTTP Response
↓
Browser Rendering

1. Browser Checks Cache First

Before contacting any server, the browser checks whether it already knows the IP address.
It checks:

✔ Browser cache
✔ OS DNS cache
✔ Router cache
✔ ISP cache
If found:

Use cache IP address

This avoids unnecessary DNS lookups and makes websites load faster.

2. DNS Resolution Happens

Humans remember names. Computer communicates using IP addresses.

So the browser asks:

What is the IP address of google.com?

The DNS server responds with something like:

142.250.xxx.xxx

This process is called DNS Resolution.

3. TCP Connection is Established

Now the browser knows the server IP.

Next step:

🤝 TCP 3-Way Handshake

Client → SYN
Server → SYN-ACK
Client → ACK

Now a reliable connection is established.

📌 Why TCP?

TCP guarantees:

✔ Reliable delivery
✔ Ordered packets
✔ Error checking
✔ Retransmission

This is extremely important for web applications.

4. TLS/SSL Handshake (HTTPS)

Since the URL starts with:

https://

A secure encrypted connection must be created.

During TLS handshake:

✔ Certificate validation
✔ Encryption negotiation
✔ Session key exchange
✔ Secure communication setup

Now communication becomes encrypted.

5. Browser Sends HTTP Request

Now the browser sends:

GET / HTTP/1.1
Host: google.com

The request also contains:

✔ Headers
✔ Cookies
✔ Authentication tokens
✔ User-Agent
✔ Compression support

6. Request Travels Across the Internet

The request moves through multiple layers:

Browser

→ Operating System
→ Router
→ ISP
→ Internet
→ CDN / Load Balancer
→ Web Server

In modern production systems:

Requests usually hit CDN or Load Balancer first

7. Server Processes the Request

Now the backend application starts working.

Possible operations:

✔ Run application logic
✔ Authenticate user
✔ Query databases
✔ Read cache
✔ Call microservices
✔ Fetch files

The server then prepares a response.

8. Server Sends HTTP Response

Example response:

HTTP/1.1 200 OK

The server may return:

✔ HTML
✔ CSS
✔ JavaScript
✔ Images
✔ JSON data

9. Browser Renders the Webpage

Now the browser starts rendering.

Internally it:

✔ Parses HTML
✔ Builds DOM tree
✔ Downloads CSS
✔ Executes JavaScript
✔ Paints pixels on screen

Finally:

🎉 The webpage appears

🎯 Interview Keywords You Must Know

DNS

TCP Handshake
TLS Handshake
HTTP Request/Response
Caching
CDN
Load Balancer
Browser Rendering
Latency
Microservices

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