Changing Host IP Address in Flask

Introduction

By default, when you run a Flask application, it starts on localhost or 127.0.0.1. This means it can only be accessed from the machine it's running on. But what if you want to access your Flask app from another device on the same network? To do that, you need to change the host IP address to make the app externally visible.

Why Change the Host IP in Flask?

Changing the host allows your Flask app to listen on different IP addresses. This is useful when:

• You want to test the app on mobile or other devices connected to the same Wi-Fi.
• You’re deploying the app on a local server.
• You're collaborating in a local network environment.

Default Flask Run Configuration

If you run Flask with app.run() and don’t specify the host, it binds to 127.0.0.1 by default:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return 'Hello from Flask!'

if __name__ == '__main__':
    app.run()  # Runs on http://127.0.0.1:5000

This version is only accessible from your local machine.

Making Flask Accessible Over the Network

To allow other devices on your local network to access the app, change the host to '0.0.0.0'. This tells Flask to listen on all available network interfaces:

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

Now your Flask app is accessible from other devices using your machine's IP address, e.g., http://192.168.1.5:5000.

Finding Your Local IP Address

To connect to your Flask app from another device, you’ll need your computer’s local IP. You can find it using the following commands:

• Windows: ipconfig (look for IPv4 Address)
• Linux/macOS: ifconfig or ip a

Running Flask with Environment Variables

You can also run the app from the terminal using the flask command with custom host and port:

# Set the Flask app file
export FLASK_APP=app.py

# Run the server on 0.0.0.0 and custom port
flask run --host=0.0.0.0 --port=8080

On Windows, use set instead of export.

Important Security Note

Exposing your app to the network (especially on public networks) can pose security risks. Always use caution and never expose the Flask development server to the internet in production environments. For production, use a WSGI server like Gunicorn or uWSGI behind a reverse proxy like Nginx.

Summary

Changing the host IP address in Flask is simple and useful for network testing and multi-device development. By setting the host to '0.0.0.0', you allow your app to accept requests from other devices on the same network. Just be sure to use this configuration wisely and securely.