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TCP - Ports

TCP uses the notion of port numbers to identify sending and receiving applications. Each side of a TCP connection has an associated 16-bit port number assigned to the sending or receiving application.
Ports are categorized into three basic categories: well known, registered and dynamic. Well known ports are assigned by the Internet Assigned Numbers Authority (IANA) and are typically used by system-level processes. Well known applications running as servers use these ports. Some examples include: FTP (21), TELNET (23), SMTP (25) and HTTP (80). Registered ports are typically used by services that have been registered by a third party. Dynamic ports can be used by end user applications.
Internally, in the operating system a socket forms the endpoint of the communication. It is defined as the combination of an IP address, a protocol, and the port number.