851 Unassigned - System Port

851 is a System Port. Transport Protocol , 851 Service Name , Service Name Description: Unassigned.

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

Port	Service Name	Description
1304	boomerang	Boomerang
1982	estamp	Evidentiary Timestamp
2208	hpiod	HP I/O Backend
2960	dfoxserver	DFOXSERVER
2977	ttc-etap-ns	TTCs Enterprise Test Access Protocol - NS
5057	intecom-ps2	Intecom Pointspan 2
5297		Unassigned
5303	hacl-probe	HA cluster probing
5520		Unassigned
5600	esmmanager	Enterprise Security Manager

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