721 Unassigned - System Port

721 is a System Port. Transport Protocol , 721 Service Name , Service Name Description: Unassigned.

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

Port	Service Name	Description
651	ieee-mms	IEEE MMS
751	pump
2403	taskmaster2000	TaskMaster 2000 Web
2907	webmethods-b2b	WEBMETHODS B2B
3010	gw	Telerate Workstation
3702	ws-discovery	Web Service Discovery
4460		Unassigned
4900	hfcs	HyperFileSQL Client/Server Database Engine
4957		Unassigned
5255		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