5621 Unassigned - User Port

5621 is a User Port. Transport Protocol , 5621 Service Name , Service Name Description: Unassigned.

Port	Protocol	Service Name	Description	Source
5621			Unassigned	IANA

Port	Service Name	Description
920		Unassigned
1314	pdps	Photoscript Distributed Printing System
1986	licensedaemon	cisco license management
2426		Unassigned
2645	novell-ipx-cmd	Novell IPX CMD
2655	unglue	UNIX Nt Glue
3546		Unassigned
3566	quest-data-hub	Quest Data Hub
4454	nssagentmgr	NSS Agent Manager
4930		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