localhost:4000: what runs on it and how to open it
Port 4000 is the default for Jekyll, Phoenix and Apollo Server. How to open it and fix errors.
Open this address: http://localhost:4000This opens on your own computer. It only works if the program is running.
localhost:4000 is the default address for the Jekyll site server, Phoenix apps and a standalone Apollo Server. It only works while the server runs in a terminal on your computer.
What runs on localhost:4000
- Jekyll with
bundle exec jekyll serve - Phoenix (Elixir) with
mix phx.server - Apollo Server (GraphQL) when started with its default port
Commands to start and test port 4000
bundle exec jekyll servebundle exec jekyll serve --port 4001mix phx.serverFind what is using port 4000
Run these in a terminal on your computer. Nothing is sent to this site.
If localhost:4000 will not open
- Port 4000 is already in use
- Another program is on port 4000. Find it with the tool below and stop it, or start yours on another port.
- Jekyll: Could not locate a Gemfile
- You started Jekyll outside your site folder. Open the folder with the _config.yml file and run the command again.
- 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 4000
| 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:4000
- What runs on localhost:4000?
- Most often a Jekyll site, a Phoenix app or an Apollo GraphQL server. The terminal shows which one.
- How do I change Jekyll's port?
- Add --port 4001 to the command, or set port in _config.yml.
- Is localhost:4000 safe?
- Yes, as long as it stays on localhost. Only your own computer can reach localhost:4000. 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 4000?
- 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:4001.
- How do I check what is using port 4000?
- 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:4000 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 :4000 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.