Why is my port closed?

To find why a port is closed, examine the transport or network path. Separate the local socket, IP delivery, TCP or UDP behaviour, firewall filtering and NAT. A successful ping does not prove a TCP port is reachable, and an open port does not prove the application protocol is healthy.

How to find where the connection stops

Work from near to far: process and bind address → host firewall → route → NAT/port forwarding → CGNAT or ISP filtering → external test. For packet-size failures, consider MTU and Path MTU Discovery separately. Always interpret a port number together with its transport protocol.

The detail that matters

A TCP RST usually means the target was reached but no listener accepted that connection; a timeout can indicate filtering, routing, NAT, or silent drop. Confirm the service binds the intended interface before changing firewall rules. To check the connection from outside your local setup, use 2ip port checker.