Port

You might say

The local app runs on port 3000 and the API on 4000. Help me confirm which one the browser reaches.

A port is a numbered network entry that distinguishes services on the same host.After npm run dev, a page commonly opens at http://localhost:3000, while an API on the same machine may listen on 4000. localhost means the current device only; ports keep multiple local services separate.

Know first

URL

Port Number

How one computer runs a page and API together

localhost:3000

This computerlocalhost

:3000Web dev serverReturns the page

:8080Local APIHandles /api requests

:5432PostgreSQLAccepts app connections

Port 3000 selects Web dev server.

What the port in an address does

The host identifies a machine: localhost identifies the computer you are using, while a deployed domain identifies a publicly reachable server. It does not identify a particular service by itself.

The port identifies the service: The number after the colon directs the request to that service. If a dev server reports 3001 as occupied or switches to 3002, the browser address must change too.

Quick checkChoose the best answer

The terminal says the page is running at `http://localhost:3001`, but you keep opening `localhost:3000` and see an old page. What should you do first?

Open the exact `http://localhost:3001` reported by the terminal, then confirm the page and API portsReplace localhost with a production domain so the port issue disappearsAssume browser cache is broken without reading the terminal

You can say this to an AI Agent

Run `npm run dev` in the project terminal and show me the complete local address from its output. If port 3000 is occupied, state the final port; then open the page and API addresses separately to confirm they reach the intended services.

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