504 citadel - System Port | Service Name:citadel

504 is a System Port. Transport Protocol TCP UDP , 504 Service Name citadel, Service Name Description: citadel.

Port	Protocol	Service Name	Description	Source
504	TCP UDP	citadel	citadel	IANA
504	TCP UDP		Citadel—multiservice protocol for dedicated clients for the Citadel groupware system,Official	wikipedia
504	TCP UDP	citadel	citadel	bekkoame

Port	Service Name	Description
1681	sd-elmd	sd-elmd
2085	ada-cip	ADA Control
2208	hpiod	HP I/O Backend
2363	mediacntrlnfsd	Media Central NFSD
2497	quaddb	Quad DB
3438	spiral-admin	Spiralcraft Admin
3988	dcs-config	DCS Configuration Port
4597	a21-an-1xbs	A21 (AN-1xBS)
5310	outlaws	Outlaws
6168		Unassigned

A port is an endpoint of communication in an operating system. While the term is also used for hardware devices, in software it is a logical construct that identifies a specific process or a type of service.

Port numbers are assigned in various ways, based on three ranges: System Ports (0-1023), User Ports (1024-49151), and the Dynamic and/or Private Ports (49152-65535);

The Well Known Ports are assigned by the IANA and on most systems can only be used by system (or root) processes or by programs executed by privileged users. Ports are used in the TCP [RFC793] to name the ends of logical connections which carry long term conversations. For the purpose of providing services to unknown callers, a service contact port is defined. This list specifies the port used by the server process as its contact port. The contact port is sometimes called the "well-known port". To the extent possible, these same port assignments are used with the UDP [RFC768]. The range for well-known ports managed by the IANA is 0-1023.
DYNAMIC AND/OR PRIVATE PORTS Registered Ports are those from 1024 through 49151.
The Dynamic and/or Private Ports are those from 49152 through 65535