Pick the port — who owns the number, and which protocol carries it
A port is sixteen bits: 0 to 65,535. ICANN cuts that range into three, and the range a number falls in tells you who chose it and what it is for. Then one further question decides the protocol: what hurts more — waiting, or losing?
the port number map
bitsarebits.com · CSE 316 — Data Communication & Networking · port ranges and the UDP/TCP choice