731 IBM NetView DM/6000 receive/tcp - System Port

731 is a System Port. Transport Protocol TCP UDP , 731 Service Name netviewdm3, Service Name Description: IBM NetView DM/6000 receive/tcp.

Port	Protocol	Service Name	Description	Source
731	TCP UDP	netviewdm3	IBM NetView DM/6000 receive/tcp	IANA
731	TCP UDP		Mac OS X RPC-based services, Used by NetInfo, for example	Apple.com
731	TCP UDP	netviewdm3	IBM NetView DM/6000 receive tcp	bekkoame

Port	Service Name	Description
703		Unassigned
1181	3comnetman	3Com Net Management
1647	rsap	rsap
2880	synapse	Synapse Transport
2883	ndnp	NDNP
4365		Unassigned
4532		Unassigned
4559	hylafax	HylaFAX
4581		Unassigned
4803	notateit	Notateit Messaging

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