localhost:4321: what runs on it and how to open it
Port 4321 is the default for the Astro dev server. How to open it and fix errors.
Open this address: http://localhost:4321This opens on your own computer. It only works if the program is running.
localhost:4321 is the default address of the Astro development server. It runs while you keep npm run dev open in a terminal.
What runs on localhost:4321
- Astro with
npm run devorastro dev
Commands to start and test port 4321
npm run devnpx astro dev --port 4322npx astro dev --hostFind what is using port 4321
Run these in a terminal on your computer. Nothing is sent to this site.
If localhost:4321 will not open
- Port 4321 is already in use
- Another program is on port 4321. Find it with the tool below and stop it, or start yours on another port.
- Port 4321 is in use, trying another
- Astro usually picks the next free port and prints the new address. Open the one in the terminal.
- This site can’t be reached (ERR_CONNECTION_REFUSED)
- The server is not running. Start it and keep that terminal open. See the full checklist.
- Works on your computer, not on your phone
- Start the server on all network addresses and use your computer’s IP address. See how to open localhost from your phone.
Other ports like 4321
| Port | Usually used by |
|---|---|
| localhost:3000 | React, Next.js, Express |
| localhost:5173 | Vite dev server |
| localhost:4200 | Angular dev server |
| localhost:5000 | Flask |
| localhost:8000 | Django, PHP, Python |
| localhost:8080 | Tomcat, Spring Boot, Docker |
| localhost:8888 | Jupyter Notebook |
Questions about localhost:4321
- What runs on localhost:4321?
- The Astro development server.
- How do I open it from my phone?
- Start it with --host so it listens on your network, then open your computer's IP address with :4321.
- Is localhost:4321 safe?
- Yes, as long as it stays on localhost. Only your own computer can reach localhost:4321. It becomes reachable from other devices only if you start the program on 0.0.0.0 or your computer's IP address. Development servers are for testing, so do not put them on the public internet.
- Can I use a different port instead of 4321?
- Yes. Most programs let you choose the port with a setting or a command option. Pick a free port above 1024 and open that number instead, for example localhost:4322.
- How do I check what is using port 4321?
- Use the port checker on this page. It shows the command for Windows, macOS and Linux and does not send anything to this site.
- Can I open localhost:4321 from my phone?
- Yes, if both are on the same Wi-Fi. Start the program so it listens on all addresses, then open your computer's IP address with :4321 on the phone. A firewall on the computer can still block it.
Free tools that help
- Common ports listSearch a list of common TCP and UDP port numbers: SSH 22, HTTP 80, HTTPS 443, MySQL 3306, PostgreSQL 5432 and more, with dev server ports.