821 Unassigned - System Port

821 is a System Port. Transport Protocol , 821 Service Name , Service Name Description: Unassigned.

Port	Protocol	Service Name	Description	Source
821			Unassigned	IANA
821	TCP UDP		Mac OS X RPC-based services, Used by NetInfo, for example	Apple.com

Port	Service Name	Description
220	imap3	Interactive Mail Access Protocol v3
842		Unassigned
1311	rxmon	RxMon
1344	icap	ICAP
2504	wlbs	WLBS
3139	incognitorv	Incognito Rendez-Vous
4598	a16-an-an	A16 (AN-AN)
5722	msdfsr	Microsoft DFS Replication Service
5942		Unassigned
8398		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