<P> The software port is always associated with an IP address of a host and the protocol type of the communication . It completes the destination or origination network address of a message . Ports are identified for each protocol and address combination by 16 - bit unsigned numbers, commonly known as the port number . </P> <P> Ports provide a multiplexing service for multiple services or multiple communication sessions at one network address . Specific port numbers are commonly reserved to identify specific services . The lowest numbered 1024 port numbers are called the well - known port numbers, and identify the historically most commonly used services . In the client--server model of application architecture, a multiplexing service is established, so that multiple simultaneous communication sessions may be initiated for the same service . The most commonly used protocols that use ports are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). </P> <P> Transport layer protocols, such as the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP), transfer data using protocol data units (PDUs). For TCP, the PDU is a segment, and a datagram for UDP . Both protocols use a header field for recording the source and destination port number . A port number is a 16 - bit unsigned integer, thus ranging from 0 to 65535 . For TCP, port number 0 is reserved and cannot be used, while for UDP, the source port is optional and a value of zero means no port . A process associates its input or output channels via an Internet socket, which is a type of file descriptor, with a transport protocol, an IP address, and a port number . This is known as binding, and enables the process to send and receive data via the network . The operating system's networking software has the task of transmitting outgoing data from all application ports onto the network, and forwarding arriving network packets to processes by matching the packet's IP address and port number . For TCP, only one process may bind to a specific IP address and port combination . Common application failures, sometimes called port conflicts, occur when multiple programs attempt to use the same port number on the same IP address with the same protocol . </P> <P> Applications implementing common services often use specifically reserved well - known port numbers for receiving service requests from clients . This process is known as listening, and involves the receipt of a request on the well - known port and establishing a one - to - one server - client dialog, using the same local port number . Other clients may continue to connect to the listening port; this works because a TCP connection is identified by a tuple consisting of the local address, the local port, the remote address, and the remote port . The well - known ports are defined by convention overseen by the Internet Assigned Numbers Authority (IANA). The core network services, such as the World - Wide Web, typically use well - known port numbers . In many operating systems special privileges are required for applications to bind to these ports, because these are often deemed critical to the operation of IP networks . Conversely, the client end of a connection typically uses a high port number allocated for short term use, therefore called an ephemeral port . </P>

What is the use of port numbers in a tcp connection
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