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A port number is the logical address of each application or process that uses a network or the Internet to communicate. A port number uniquely identifies a network-based application on a computer. Each application/program is allocated a 16-bit integer port number. This number is assigned automatically by the OS, manually by the user or is set as a default for some popular applications.
A port number primarily aids in the transmission of data between a network and an application. Port numbers work in collaboration with networking protocols to achieve this. For example, in an incoming message/packet, the IP address is used to identify the destination computer/node, whereas the port number further specifies the destination application/program in that computer. Similarly, all outgoing network packets contain application port numbers in the packet header to enable the receiver to distinguish the specific application.
Port numbers are mainly used in TCP and UDP based networks, with an available range of 65,535 for assigning port numbers. Although an application can change its port number, some commonly used Internet/network services are allocated with global port numbers such as Port Number 80 for HTTP, 23 for Telnet and 25 for SMTP.