Port
The local app runs on port 3000 and the API on 4000. Help me confirm which one the browser reaches.
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.localhost:3000:3000Web dev serverReturns the page:8080Local APIHandles /api requests:5432PostgreSQLAccepts app connectionsPort 3000 selects Web dev server.
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.
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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