8491 Unassigned - User Port

8491 is a User Port. Transport Protocol , 8491 Service Name , Service Name Description: Unassigned.

Port	Protocol	Description	Source
8491		Unassigned	IANA
8491	TCP UDP	EMC2 (Legato) Networker or Sun Solstice Backup,Official	wikipedia
8491	UDP	QuickTime Streaming Server	Apple.com
8491	TCP	Web service, iTunes Radio streams	Apple.com

Port	Service Name	Description
135	epmap	DCE endpoint resolution
727		Unassigned
1563	cadabra-lm	Cadabra License Manager
1787	funk-license	funk-license
1806	musiconline	Musiconline
2691	itinternet	ITInternet ISM Server
3742	cst-port	CST - Configuration & Service Tracker
4040	yo-main	Yo.net main service
6695		Unassigned
6859		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