728 Unassigned - System Port

728 is a System Port. Transport Protocol , 728 Service Name , Service Name Description: Unassigned.

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

Port	Service Name	Description
574	ftp-agent	FTP Software Agent System
1141	mxomss	User Message Service
2527	iqserver	IQ Server
5510		Unassigned
5819		Unassigned
5839		Unassigned
7332	swx	The Swiss Exchange
7978		Unassigned
9111		Unassigned
9159		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