Port 721 Unassigned - System Port

Port 721 is a System Port. Transport Protocol , Port 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
1491		Unassigned
2161	apc-2161	APC 2161
2640	sai-sentlm	Sabbagh Associates Licence Manager IANA assigned this well-formed service name as a replacement for "sai_sentlm".
3385	qnxnetman	qnxnetman
4802	ilss	Icona License System Server
4877	lmcs	Lighting Management Control System
5023	htuilsrv	Htuil Server for PLD2
5790		Unassigned
7567		Unassigned
9253		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