Common service ports and what they mean

Common service ports belong to 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

Common defaults are 22/TCP for SSH, 25/TCP for SMTP, 53/UDP and 53/TCP for DNS, 80/TCP for HTTP and 443/TCP for HTTPS. A default port is a convention, not proof of the running application, and services can listen on nonstandard ports. To check the connection from outside your local setup, use 2ip server check.