A well-known port is a TCP or UDP port numbered 0–1023. IANA strictly controls this range and reserves it for common protocols, also calling these system ports. section 22.1.1 section 22.3
Recognize well-known ports by their protocol assignments:
- TCP 22 carries SSH, TCP 80 carries HTTP, and TCP 443 carries HTTPS. section 22.1.1
- UDP 53 supports DNS, while UDP 67 and 68 support DHCP server and client traffic. section 22.1.1
- Do not confuse this range with registered ports 1024–49151 or ephemeral ports 49152–65535, commonly selected by clients as source ports. section 22.1.1
Client-to-server connection
A PC can send a TCP segment from ephemeral source port 50000 to an HTTPS server at destination port 443. The reply reverses the ports: source 443 and destination 50000. section 22.1.1
