localhost:3000

What the address means

In http://localhost:3000/login:

• http is the application protocol.
• localhost resolves to the current device’s loopback interface, commonly 127.0.0.1 or ::1.
• 3000 selects one TCP port on that device.
• /login is an application path sent after the connection succeeds.

Why port 3000 appears in development

Port 3000 is a convention used by several tools and tutorials. Next.js documents it as the default for next dev. A Node or Express application can listen on 3000 when its code or environment selects that value. Other tools use different defaults: Vite documents 5173, Django uses 8000 for runserver, and Flask uses 5000 for its development command.

The terminal output is more reliable than the convention. If a tool prints 3001, 5173, or another URL, use the printed address.

Start and verify a service

For an existing project, use the command in its README or package scripts. A common Next.js project uses:

npm run dev
# then open the exact Local URL printed by Next.js

A minimal Node HTTP server can make the relationship explicit:

// server.mjs
import http from 'node:http'

http.createServer((request, response) => {
  response.writeHead(200, { 'content-type': 'text/plain' })
  response.end(`You requested ${request.url}\n`)
}).listen(3000, '127.0.0.1')

node server.mjs
curl -i http://127.0.0.1:3000/

Why /login, /admin, or /docs may fail

Port 3000 only gets the request to one server. That server must still recognize the path and method.

A browser address bar makes a GET request. An API route that accepts only POST will not behave like a web page.

Check what is actually running

# Windows PowerShell
Get-NetTCPConnection -LocalPort 3000 -State Listen |
  Select-Object LocalAddress, OwningProcess

# macOS or Linux
lsof -nP -iTCP:3000 -sTCP:LISTEN

# Inspect the HTTP response
curl -i http://127.0.0.1:3000/

No listener means there is nothing to fix in the browser. An HTTP response means the connection works and the status, headers, application log, and browser console should guide the next step.

For a full decision tree covering ERR_CONNECTION_REFUSED, EADDRINUSE, Docker, WSL, and blank screens, use localhost:3000 not working.

When to use another port

Two services cannot normally listen on the same address and port at the same time. Configure one service on a different port when projects run together. The port number itself does not change whether a service is a frontend, API, database, or admin interface.

Official references

• Next.js CLIDefault development port and port/hostname options.
• Node.js net serverHow a server binds to a port and reports address conflicts.
• MDN: Local network accessLoopback, local, and public address spaces.