Running on http://127.0.0.1:5000 Explained in Simple Terms

If you have just started a Flask project and your terminal printed a line like “Running on http://127.0.0.1:5000”, congratulations — that line is good news, not an error. It is the development server’s way of telling you that your application started successfully and is now waiting for you to visit it. Open http://127.0.0.1:5000 in your browser, and you should see your app. If you see a “can’t reach this site” page instead, something small has gone wrong between the server and the browser, and this guide will help you find it.

New developers often stare at that terminal line and wonder whether they are supposed to do something with it, whether the address is a real website, or whether the message means the app is already public. None of those worries apply. The address points to your own computer, the app is only visible to you, and the only action the message asks of you is to open the address in a browser and start testing. The rest of this guide explains each part of the message, what to do with it, how to change the port or address when you need to, and how to fix the handful of problems that commonly appear around it.

In this article:

• What “Running on http://127.0.0.1:5000” actually means
• The message, piece by piece
• What to do right after you see it
• Why the page sometimes will not load
• How to change the port or the host address
• The debugger and reloader, in plain language
• “Address already in use” on port 5000
• Can someone else on your network see your app?
• Why you should not use this server for a real website
• Frequently asked questions

What “Running on http://127.0.0.1:5000” Actually Means

In one sentence: your Flask application is running on your own computer, and a small built-in web server is serving it on port 5000, reachable only from that same computer unless you deliberately change the settings.

Flask is a Python web framework — a toolkit for building web applications in Python. When you run a Flask app for development, Flask starts its own built-in development server, a lightweight program whose only job is to take requests from a browser and hand them to your application code. That development server is convenient because it requires no extra setup: you write your app, run one command, and you have a working local website to test against. It is not, however, a production server, a distinction that matters later and is covered in its own section below.

Key: 127.0.0.1. The loopback address, meaning “this computer.” Traffic sent here never leaves the machine.

Key: port 5000. The port Flask’s development server uses by default, unless you configure a different one.

Key: development server. A lightweight server for local testing, included with a framework. It is built for convenience, not for public traffic.

The message appears in your terminal because that is where the server runs. As long as the terminal window stays open and the server process keeps running, your app stays available at that address. Close the terminal, stop the process, or shut down your computer, and the address stops working — not because anything broke, but because the server that answered there is no longer running. That is the single most important thing to understand about the message: it describes a live process on your machine, not a permanent website. When the process ends, the address goes quiet.

The Message, Piece by Piece

“Running on” is the server announcing that startup succeeded. If startup had failed — because of an error in your code, a missing file, or a port conflict — you would see an error message instead of this line, usually with a traceback pointing at the problem. The presence of the “Running on” line is therefore a useful health signal: your code at least loaded and the server managed to bind to its port.

“http://” tells you the server speaks plain HTTP, not encrypted HTTPS. That is normal for local development. Your browser will open the address without any security warning about certificates, because there is no certificate involved. Do not try to force HTTPS here; there is no secure server listening, and an HTTPS request to a plain HTTP server will fail.

“127.0.0.1” is the loopback address. It means the server is listening for connections from your own computer only. Some Flask setups print “localhost” instead of the numeric address; the two are equivalent in practice, because localhost normally resolves to 127.0.0.1. If you see one form in a tutorial and the other in your terminal, do not worry — they point to the same place.

“5000” is the port. Flask’s development server defaults to port 5000, which is why the number shows up so often in Flask tutorials and error reports. It is a default, not a law: you can run Flask on another port whenever you need to, and the message will then print that port instead. The port is also why the full address matters. Typing just 127.0.0.1 into a browser, without a port, sends the request to the default web port, where nothing is listening, and you will get a failure even though your app is running fine on 5000. Always include the port.

Finally, the message is sometimes followed by a note telling you to press a key combination to quit the server. That is your reminder that the server keeps running until you stop it deliberately. Keeping that terminal window visible while you develop is good practice: it shows you each request your browser makes, along with any errors your code produces, and that running log is one of the most useful debugging tools you have.

What to Do Right After You See It

The next step is simple: open the address in your browser. Clicking the address in some terminals opens it directly; otherwise, copy it into your browser’s address bar. You should see whatever your application returns for its home page — for a brand-new project, that might be a simple greeting or a starter page, depending on how the project was set up.

Then interact with the app the way a visitor would. Click its links, submit its forms, and watch the terminal as you do. Each request appears as a log line, usually showing the request method, the path, and a status code. A status in the 200 range means the request succeeded. A 404 means your app has no route for that path — you asked for a page the app does not define. A 500 means your code raised an error while handling the request, and the terminal will usually show the details. These log lines turn vague “it doesn’t work” feelings into specific, fixable facts, so make a habit of glancing at the terminal whenever the browser surprises you.

One more habit worth building early: keep the server terminal and the browser side by side while you develop. When you edit your code and save, the development server’s reloader — covered below — usually restarts the app automatically, and you can refresh the browser to see the change. If a change breaks the app, the terminal tells you immediately, while the cause is still fresh in your mind. Developers who hide the terminal and only check it after a long string of failures have a much harder time tracing what went wrong.

Why the Page Sometimes Will Not Load

If the terminal says the server is running but the browser says it cannot reach 127.0.0.1:5000, work through these causes in order.

First, check that you typed the full address, including the port. This is the most common beginner mistake, and it is an easy one to make, because browsers hide port numbers for normal websites. For local development, the port is part of the address. A browser pointed at http://127.0.0.1 without a port is knocking on a different door, and nobody is home there.

Second, check that the server is still running. Look at the terminal: is the process still alive, or did it stop with an error after printing the “Running on” line? A server can start successfully and then crash on the first request if your code has a bug that only appears when a page is actually visited. If the terminal shows a traceback, that traceback — not the browser message — is the real problem to fix.

Third, try http://localhost:5000 as an alternative to the numeric address. The two should behave identically, but if one works and the other does not, you have learned something useful about name resolution on your machine. If neither works, the problem is with the server or the connection, not the name.

Fourth, try a private browser window. Cache, extensions, and stale redirects can all interfere with local addresses. A private window rules those out in one test. If the private window works, clear site data for 127.0.0.1 in your normal browser and check extensions.

Fifth, check for a proxy or VPN interfering with local traffic. Local addresses should bypass any proxy, but a misconfigured proxy setting can send even loopback traffic outward, where it fails. Make sure localhost and 127.0.0.1 are on the proxy bypass list, and try briefly disconnecting a VPN as a test. If the page loads with the VPN off, the VPN’s routing is the culprit.

How to Change the Port or the Host Address

Changing the port is straightforward, and there are two common ways to do it. The first is at the command line: the command that starts the server usually accepts a port option, so you can start it on a different port without touching your code. The second is in code: the call that runs the application accepts port and host settings, and you can set them there. Either way, after restarting, the terminal will print a new “Running on” line with the new address, and that new line is the truth — open it, not the old one.

Why would you change the port? The usual reason is a conflict: something else already uses port 5000. Another reason is running two Flask projects at once, which requires two different ports. A third is personal preference or a team convention. Whatever the reason, pick a free port above 1024, verify nothing is listening on it, and update any bookmarks or front-end code that calls the local address.

Changing the host address is a different decision with security implications. By default, Flask’s development server listens on 127.0.0.1, meaning only your own computer can reach it. If you change the host to 0.0.0.0, the server listens on all network interfaces, which means other devices on your network can reach it using your computer’s network address. That is occasionally useful — for example, to preview your app on your phone — but it also exposes your development server to your whole network. Do it deliberately, for a short test, and switch back when you are done.

Never change the host casually just because a forum post suggested it, and never forward a development port through your router to the public internet.

The Debugger and Reloader, in Plain Language

Two features of the development server confuse newcomers, so they are worth a plain explanation. The reloader watches your code files for changes. When you save an edit, it restarts the application automatically, so you can refresh the browser and see the new version without manually stopping and starting the server. If you ever wonder why the terminal printed a fresh “Running on” line without you doing anything, the reloader is why. It also explains a common annoyance: if you edit a file while the server is running and introduce a syntax error, the restart fails and the server may stop serving until you fix the error and save again. The terminal will show what went wrong.

The debugger is a development aid that shows detailed error information when your code fails. In its most interactive form, it can present an error page in the browser with a console for inspecting the failure. That is enormously helpful while you are learning, because it shows you exactly where the error happened and what the values were. It is also dangerous outside local development: an interactive debugger exposed to a network lets anyone who reaches it run code on your machine.

That is one of the reasons the development server must never be used as a public server. Use the debugger happily on 127.0.0.1, where only you can reach it, and never enable it on a server that other people can access.

A practical tip follows from both features: when something behaves strangely after an edit, check whether the reloader actually restarted cleanly. A failed reload leaves the old code running or no code running at all, and the symptom — a page that does not reflect your latest edit, or a page that suddenly fails — looks like a bug in your new code when it is really a restart problem. The terminal log tells you which.

“Address Already in Use” on Port 5000

Port 5000 is popular beyond Flask, and that popularity causes collisions. System services, other development tools, and media software have all been known to claim port 5000 on various machines. When Flask cannot start because the address is already in use, the error is telling you that another program got there first. Only one program can listen on a given port at a time.

Your options are the same as for any port conflict. Find the current holder with your system’s port-listing tool — netstat on Windows, lsof or ss on macOS and Linux — and decide. If the holder is a leftover Flask server from an earlier session, stop it and start fresh; it may even still be serving an old version of your project. If the holder is something you need, move Flask to another port instead. If

If you cannot identify the holder, or you cannot stop it, moving Flask to a free port is the path of least resistance and costs you nothing: your app works exactly the same on port 5001 or 8000 as it does on 5000, as long as you open the address the server actually prints.

Do not fall into the trap of restarting your computer every time this happens. A restart clears the port, but if the holder is a program that starts automatically, the conflict returns on the next boot. Finding the holder once is a permanent lesson; rebooting is a temporary one.

Can Someone Else on Your Network See Your App?

Only if you let them. With the default host of 127.0.0.1, the answer is no: the server accepts connections only from the same machine, so a colleague on the same Wi-Fi network cannot open your app, and neither can anyone on the internet. Your app is private by default, which is exactly how a development setup should behave.

If you need someone else to see it — a teammate reviewing a design, or yourself testing on a phone — you have two safe choices. The first is to bind the server to all interfaces temporarily and share your computer’s local network address with the port, as described above, keeping in mind that everyone on that network can then reach it.

The second, safer for anything beyond a quick look, is to deploy the app properly to a staging environment or a hosting service, where access control, encryption, and a production server are in place. For a quick phone preview on your own home network, the temporary binding is fine. For sharing with people outside your network, deployment is the right answer, not port forwarding a development server.

Why You Should Not Use This Server for a Real Website

This point deserves emphasis because the development server makes launching a site feel deceptively easy. It is tempting to leave it running and point visitors at it. Resist that temptation. The development server is designed for one developer testing locally. It handles requests one at a time in a simple way, it is not hardened against hostile traffic, it has no built-in protections that production servers provide, and with debugging features enabled it can expose dangerous capabilities to anyone who reaches it. It will also slow down or fail under real traffic, and it will stop the moment you close its terminal or restart your computer.

A real deployment uses a production-grade server, a proper process manager that restarts the app if it crashes, encryption for visitor traffic, and a hosting environment that stays up independently of your laptop. Learning that deployment process is a separate step from learning Flask itself, and it is worth doing properly when your app is ready for other people. Until then, the “Running on” message is your private workshop, not your shopfront.

Frequently Asked Questions

What does “Running on http://127.0.0.1:5000” mean?

It means your Flask development server started successfully on your own computer and is serving your app on port 5000. Open that address in your browser to see your app.

Is my Flask app public when I see this message?

No. With the default settings, the app is reachable only from your own computer. Nobody else can open 127.0.0.1:5000 and see your app, because that address always refers to the visitor’s own machine.

Why does the page not load even though the terminal says it is running?

Check the full address including the port, confirm the server process is still alive, try a private browser window, and check for proxy or VPN interference. Also read the terminal log: a crash on the first request looks like a connection failure in the browser but shows a clear error in the terminal.

How do I stop the Flask development server?

Stop the process in its terminal, using the quit key combination the server prints, or by closing the terminal. Once the process ends, the address stops responding, which is expected.

How do I run Flask on a different port?

Start the server with a different port option, or set the port in the code that runs the app, then restart. The terminal will print a new “Running on” line with the new port — open that address.

What does “address already in use” mean for port 5000?

Another program is already listening on port 5000. Find it with a port-listing tool and stop it, or run Flask on a different port.

Can I open my Flask app from my phone?

Yes, temporarily, by binding the server to all interfaces and opening your computer’s local network address with the port from your phone on the same network. Remember this exposes the development server to your network, so switch the binding back when you are done.

Is the Flask development server safe to use for a live website?

No. It is built for local development only. It lacks the performance, security, and reliability features of a production server, and debugging features can be dangerous if exposed. Deploy properly when the app is ready for real visitors.

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