520 extended file name server - System Port

520 is a System Port. Transport Protocol TCP , 520 Service Name efs, Service Name Description: extended file name server.

Port	Protocol	Service Name	Description	Source
520	TCP	efs	extended file name server	IANA
520	UDP	router	local routing process (on site); uses variant of Xerox NS routing information protocol - RIP	IANA
520	TCP		efs, extended file name server,Official	wikipedia
520	UDP		Routing Information Protocol (RIP),Official	wikipedia
520	UDP		(UDP) - A UDP backdoor	simovits.com
520	UDP	router	local routing process (on site); uses variant of Xerox NS routing information protocol - RIP	bekkoame
520	TCP	efs	extended file name server	bekkoame

Port	Service Name	Description
612	hmmp-ind	HMMP Indication
921		Unassigned
1299	hp-sci	hp-sci
2561	mosaixcc	MosaixCC
4165	altcp	ArcLink over Ethernet
4249	vrml-multi-use	VRML Multi User Systems
4794		Unassigned
7268		Unassigned
7370		Unassigned
7628	zen-pawn	Primary Agent Work Notification

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