Port Number Reference

IANA divides the 16-bit port space into three ranges: well-known (0–1023, system ports for standardized services), registered (1024–49151, applications and vendor-assigned), and ephemeral (49152–65535, dynamic / private ports for outgoing connections). On Unix you need root to bind below 1024.

DNS, SIP, and STUN, for example, are defined for both TCP and UDP. The both marker tells you the port is registered on both protocols, not that the same service must use them simultaneously.

Yes — they're either IANA assignments (22, 80, 443, 53, …) or de-facto defaults from the upstream project (3306 MySQL, 6379 Redis, 27017 MongoDB). Implementations can listen on a different port, of course.

Barely. It removes the noise of untargeted scanners that only probe the default, which does quiet your logs, but anything scanning the full range finds the service in seconds and identifies it from its banner. Treat a moved port as log hygiene rather than as a control: authentication, key-based access and a firewall rule are what actually keep the service closed.

Because ports below 1024 are historically privileged on Unix-like systems: binding one requires root, on the reasoning that a well-known port implies a service the machine's owner sanctioned. The usual modern answers are to bind a high port and put a reverse proxy in front, or to grant the one capability that permits low binds rather than running the whole process as root.