4051 Cisco Peer to Peer Distribution Protocol - User Port

4051 is a User Port. Transport Protocol TCP UDP , 4051 Service Name cppdp, Service Name Description: Cisco Peer to Peer Distribution Protocol.

Port	Protocol	Service Name	Description	Source
4051	TCP UDP	cppdp	Cisco Peer to Peer Distribution Protocol	IANA

Port	Service Name	Description
846		Unassigned
1632	pammratc	PAMMRATC
1785	windlm	Wind River Systems License Manager
1948	eye2eye	eye2eye
2037	applus	APplus Application Server
2390	rsmtp	RSMTP
2421	g-talk	G-Talk
4069	minger	Minger Email Address Validation Service
4278	vrml-multi-use	VRML Multi User Systems
5125		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