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A network port is a process-specific or an application-specific software construct serving as a communication endpoint, which is used by the Transport Layer protocols of Internet Protocol suite, such as User Diagram Protocol (UDP) and Transmission Control Protocol (TCP).
A specific network port is identified by its number commonly referred to as port number, the IP address in which the port is associated with and the type of transport protocol used for the communication.
A port number is a 16-bit unsigned integer that ranges from 0 to 65535.
If you could consider all the addresses a computer processor could talk to as the address space, then certain addresses will have specialized purposes. For example, an address could be a memory address or another address could be a port address. A port address could be used to talk to external processes or devices. A port then, is simply a hole in the processor address space where data can be sent and received from.
Any networking process or device uses a specific network port to transmit and receive data. This means that it listens for incoming packets whose destination port matches that port number, and/or transmits outgoing packets whose source port is set to that port number. Processes may use multiple network ports to receive and send data.
The port numbers that range from 0 to 1023 are known as well-known port numbers. Well-known port numbers are allotted to standard server processes, such as FTP and Telnet. They are referenced by system processes providing widely used types of network services. Specific port numbers are assigned and recorded by the Internet Assigned Numbers Authority (IANA).
However, in common practice, there is much unofficial use of both officially assigned numbers and unofficial numbers. Additionally, some network ports are in use for multiple applications and may be designated as either official or unofficial.