Well-known port
A well-known port -- formally a System Port in RFC 6335's terminology -- is one of the 1,024 port numbers from 0-1023 that IANA reserves for an established service, such as 22 for SSH, 443 for HTTPS, or 502 for Modbus/TCP. RFC 6335 divides the full 0-65535 TCP/UDP port space into three ranges tracked in IANA's Service Name and Transport Protocol Port Number Registry: System Ports (0-1023, the well-known ports, requiring IANA review to assign), User Ports (1024-49151, also called registered ports, assignable to a named application on request), and Dynamic or Private Ports (49152-65535, never formally assigned, used as ephemeral source ports for outbound connections). None of these ranges are enforced by the network itself -- any sufficiently privileged process can bind to any port regardless of what the registry says it's for -- so a port number is a strong hint about what's listening, not a guarantee; an open port that doesn't match its registered service is itself worth investigating.
By AdminKit team, Netbase JSC · Updated