Port 3000 — Next.js / React / Nuxt / Express

1. What is Port 3000?

Port 3000 is the undisputed default development port for JavaScript full-stack ecosystems. When you run `npm run dev` in a Next.js or React application, the Node.js runtime creates a TCP listening socket bound to IPv4 `127.0.0.1:3000` or IPv6 `[::1]:3000`.

In early Node.js tutorials and Express examples (circa 2011), port 3000 was widely chosen because port numbers below 1024 require elevated root/administrator privileges on Unix systems. Port 3000 is high enough to avoid privileged access, sits well below the dynamic/ephemeral range (49152–65535), and has no conflicting core system service assignments in the official IANA registry.

2. How Port 3000 Works Under the Hood

When a Next.js development server launches on Windows: 1. `node.exe` invokes the Win32 Winsock API `socket()` followed by `bind()` to associate the socket with `0.0.0.0:3000` or `127.0.0.1:3000`. 2. It executes `listen()` with a TCP connection backlog queue. 3. Windows kernel writes an entry into the system TCP listener table (`MIB_TCPTABLE_OWNER_PID`). 4. Incoming HTTP requests trigger an `accept()` loop and are dispatched to the Next.js compilation/Webpack/Turbopack dev server. 5. Hot Module Replacement (HMR) maintains a persistent WebSocket connection over the same port to push incremental code updates to your browser.

3. Common Conflicts & 'Error: listen EADDRINUSE: address already in use :::3000'

Because port 3000 is used by hundreds of libraries (Next.js, Create React App, Nuxt, Remix, Rails puma, Puma, Grafana), developers frequently suffer from `EADDRINUSE` errors. This usually happens when: - A terminal tab or VS Code window was closed without gracefully sending `SIGINT` (Ctrl+C). - A child Node.js worker process got orphaned in the background. - Another project (e.g. Grafana or a previous client frontend) is still running. When Next.js encounters a taken port 3000, it prints `Port 3000 is in use, trying 3001 instead`. While convenient, this causes subtle bugs: third-party OAuth providers (Google, GitHub) configured with callback `http://localhost:3000/api/auth/callback` will fail with redirect URI mismatch errors!

4. How to Find & Stop the Process on Port 3000

If port 3000 is locked by an orphaned process or background service, choose one of the following methods:

Or simply use PortPeek

Never memorize netstat flags or parse PIDs again.

1. Press Win + Alt + P anywhere on Windows.

2. See Port 3000 with its owning process (node.exe) and memory usage.

3. Click Stop Process to kill it instantly, or click the item to launch in your browser.

Download PortPeek (Free & Open Source)

5. Standards & Related Ports