Docker port already allocated?

Stop the conflicting container or map host ports to a free value.

Port works once then fails?

TIME_WAIT can briefly hold ports—use SO_REUSEADDR in servers you control.

Pick another port or disable the service using that port.

Error fix

Something else is already listening on that TCP port—another dev server or a zombie process.

Start here first. Step 1 fixes most cases—then work down the list.

The OS refused to bind because only one listener can own a given port on an interface (for typical servers).

Duplicate dev servers
Previous process not killed or IDE auto-started a second server.
System services
Another app (IIS, SQL, Docker) already claimed the port.

Free or change the port
1. Identify PID with netstat/lsof/ss and stop the process or kill it safely.
2. Change your app’s port via env (PORT=3001) or framework flag.
3. On Windows, resolve System PID 4 vs HTTP.sys URLACL conflicts if applicable.

Docker port already allocated?: Stop the conflicting container or map host ports to a free value.
Port works once then fails?: TIME_WAIT can briefly hold ports—use SO_REUSEADDR in servers you control.
macOS AirPlay using 5000?: Pick another port or disable the service using that port.

Still stuck? Try these related fixes next.

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Original error message

EADDRINUSE: address already in use :::3000