Common port numbers

The ports you actually run into, with what each is for and why it matters if one is exposed.

The three port ranges

The ports that should never face the internet

If a scan finds any of these open on a public address, treat it as an incident rather than a configuration preference: 23 (Telnet), 445 (SMB), 2375 (Docker API), 3306 (MySQL), 3389 (RDP), 5432 (PostgreSQL), 6379 (Redis), 9200 (Elasticsearch) and 27017 (MongoDB). Each of these has been the root cause of large, well-documented breaches.

Why development servers use 3000 and 8080

On Unix-like systems, binding to a port below 1024 requires root. Rather than run a development server as root, the convention settled on high ports — 3000, 4200, 5173, 8000, 8080 — which any user process can bind. In production a reverse proxy holds 80 and 443 and forwards to the application on its high port.