---
title: Reserved Port
url: "https://nextjs.org/docs/messages/reserved-port"
docs_index: /docs/llms.txt
---



## Why This Error Occurred

Server was started on a reserved port. For example, `4045` is reserved for the Network Paging Protocol (npp).

```bash filename="Terminal"
next start -p 4045
```

or

```bash filename="Terminal"
next dev --port 4045
```

Starting the server on a reserved port will result in an error.

## Possible Ways to Fix It

Change the provided port to ensure it's not listed in the [Port Blocking](https://fetch.spec.whatwg.org/#port-blocking) section of WHATWG's fetch spec.

## Useful Links

- [Port blocking](https://fetch.spec.whatwg.org/#port-blocking)