EP 4: What actually happens when you type Localhost?

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Have you ever typed localhost in your browser while coding? I’m sure you must have! Whether you're a frontend developer launching a React or an Angular app or a backend engineer testing APIs, "localhost" is where your coding journey begins. But what really happens when you type http://localhost:8080?

Have you ever wondered why your React app runs on http://localhost:3000 or why your Angular apps runs on http://localhost:4200/ ? Have you ever stopped to think, what actually happens when you type localhost and hit enter? If you’re a developer, localhost is the first thing that you will witness in development.

Let’s break it down in simple terms in this blog today.

What is localhost?

To explain in layman terms, Localhost is a nickname for your own computer. In technical terms, it’s a reserved domain name that always points back to your own machine. You can think of it as your computer’s self-address. So instead of typing your computer’s actual IP address, like 127.0.0.1, you can just type localhost.

So when you type localhost into your browser, you’re not reaching out to the internet, you’re talking to your own machine.

How Does Localhost Work?

To understand localhost better, let’s go through an example of a typical website request and see,

When You Visit Google.com:

1. Your browser asks a DNS (Domain Name System) server to find Google’s IP address.

2. The request travels through the internet to reach Google’s servers.

3. Google processes the request and sends the response back to your browser.

When You Visit Localhost:

1. Your computer skips all those steps. No DNS lookup happens! Your system already knows that localhost means "this computer".

2. The request stays within your machine and is processed internally.

3. If you have a web server running, it handles the request.

127.0.0.1: The IP Behind Localhost

Localhost is simply mapped to 127.0.0.1 by the operating system itself. Also, 127.0.0.1 is already a predefined IP address that directly references the loop back interface, no lookup is required. It works because the system understands that any request to 127.0.0.1 should never leave your machine. The loop back interface is a virtual network interface that sends data right back to the same device.

The entire 127.0.0.0/8 range is reserved for loop back that means 127.0.0.1, 127.0.0.2, 127.0.0.3, 127.0.0.4,… and even127.255.255.255 all point to your machine but in practice 127.0.0.1 is the standard in short 127.0.0.1 is a hardcoded loop back IP always understood by your system and Localhost is a name that gets mapped to 127.0.0.1 via the host file.

Now if you modify your host file to map “TechMonk” to 127.0.0.1 then typing TechMonk in your browser or terminal will work just like Localhost it will resolve TechMonk and use the loopback interface.

Where is Localhost defined?

Your operating system has a special file that maps localhost to 127.0.0.1:

• Linux/macOS: /etc/hosts

• Windows: C:\Windows\System32\drivers\etc\hosts

This mapping overrides any DNS lookup and ensures that localhost always points to your machine.

Can You Change Localhost?

When you go to command line and type “ping localhost” you will get something like this:

And when you type “ping techmonk” you will get something like below,

You can map custom names to 127.0.0.1 for local testing. For example, adding this line to your hosts file:

127.0.0.1  techmonk

Now, typing techmonk in your browser will act just like localhost. But be careful, editing this file requires admin rights, and mistakes can cause network issues.

In case you want step by step procedure for windows,

To make techmonk resolve to 127.0.0.1, you need to edit the hosts file manually, not run it as a command. Follow these steps:

1. Open Notepad as Administrator

   • Press Win + S, type Notepad, right-click it, and select Run as administrator.

   • Click File > Open and navigate to:

C:\Windows\System32\drivers\etc\

• Change "Text Documents (*.txt)" to "All Files" at the bottom.

• Open the hosts file.

2. Add Your Custom Domain Entry

At the bottom of the file, add this line:

127.0.0.1 techmonk

This tells Windows that whenever you type techmonk in the browser, it should resolve to 127.0.0.1(localhost).

3. Save and Apply Changes

   • Save the file (Ctrl + S).

   • Close Notepad.

   • Open Command Prompt and run:

ipconfig /flushdns

4. Test It

   • Open a browser and type http://techmonk/ → It should point to your local server.

   • Or, open CMD and run:

ping techmonk 

Now, techmonk works like localhost, and you can access your local projects using a custom domain name.

You will get something like this,

IPv4 vs. IPv6 wrt localhost

Localhost can use two different address formats:

• IPv4: 127.0.0.1

• IPv6: ::1

You can check which one your system prefers by running:

ping localhost

If you see responses from 127.0.0.1, your system uses IPv4. If you see ::1, it’s using IPv6.

Why Port 3000?

When you run npm run dev, your React app opens on localhost:3000. But why 3000?

Different web servers use different default ports:

• Port 80 → Default for websites (HTTP)

• Port 443 → Secure websites (HTTPS)

• Port 3000 → Common for frontend development (React, Next.js)

• Port 5000 → Often used for backend APIs (Flask, Django)

• Port 8080 → Alternative for web servers

For example:

• React, Next.js: 3000

• Angular: 4200

• Node.js/Express: 3000 or 8000

• Python/Flask: 5000

• Ruby on Rails: 3000

• Apache/Nginx: 80 (HTTP) or 443 (HTTPS)

If a port is already in use, you can change it manually or check what’s occupying it using:

• Mac/Linux: lsof -i :3000

• Windows: netstat -ano | findstr :3000

If another app is already using port 3000, you might need to pick a different one.

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Why do Developers use Localhost?

Developers rely on localhost to test and debug applications before making them live. Here’s why:

1. Offline Development: You don’t need the internet to build and test websites or APIs.

2. Security: Localhost traffic never leaves your machine, making it safe for testing sensitive applications.

3. Speed: Localhost is much faster than connecting to a remote server since everything stays inside your computer.

4. Debugging: Developers can test code in a safe environment before deploying it online.

Localhost in Code

Python:

In Python, the socket.gethostbyname() function returns the IP address of the given hostname. When you pass "localhost", it resolves to 127.0.0.1, which is the loopback address for your local machine.

import socket
print(socket.gethostbyname("localhost"))  # Output: 127.0.0.1

Java:

In Java, InetAddress.getByName("localhost") resolves "localhost" to its IP address (127.0.0.1). This shows how Java connects to the system's network interface to get the loopback address.

import java.net.InetAddress;

public class LocalhostDemo {
    public static void main(String[] args) throws Exception {
        InetAddress localhost = InetAddress.getByName("localhost");
        System.out.println(localhost.getHostAddress());  // Output: 127.0.0.1
    }
}

Node.js:

In Node.js, dns.lookup() is used to resolve a hostname into an IP address. When you use "localhost", it returns 127.0.0.1 (IPv4) or ::1 (IPv6), depending on your system configuration.

const dns = require('dns');

dns.lookup('localhost', (err, address) => {
    console.log(address); // Output: 127.0.0.1 or ::1
});

Public vs. Private vs. Loopback IPs

Common Localhost Issues & Fixes

Port Already in Use

Sometimes, you might find that the port you're trying to use (like 3000) is already in use. To fix this, you can find and stop the process occupying the port:

• Linux/macOS: lsof -i :3000

• Windows: netstat -ano | findstr :3000

Once you know the PID (process ID), you can stop it:

• Linux/macOS: kill -9 <PID>

• Windows: taskkill /F /PID <PID>

Sharing Your Localhost Work

Want to share your localhost project with others? Tools like ngrok let you create a temporary public URL for your local app:

ngrok http 3000

This creates a tunnel from a public URL to your local server, allowing others to access your project even though it’s running on localhost.

Final Thoughts

Next time you see localhost:3000 , remember:

• Localhost is just a convenient alias for 127.0.0.1, your machine’s own IP address.

• It’s a safe, fast environment for testing and debugging code.

• Localhost helps you build and test software locally before deploying it live.

Understanding localhost makes you a better developer. Now, go build something awesome!

I hope this gives you a clear picture of Localhost, WHAT it is and HOW it works.

In a nutshell, Localhost is a loopback network address (127.0.0.1) that allows developers to test and debug applications locally on their own machine without the need for an internet connection. It's essential for safe, fast, and offline development.

Keep learning. You’ve got this!