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, associated with a transport protocol, an IP address, and a port number. This is known as binding. A socket is used by a 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 to a socket. 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.

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 potentially establishing a one-to-one server-client dialog, using this listening port. Other clients may simultaneously connect to the same 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). 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.

Common port numbers

IANA is responsible for the global coordination of the DNS root, IP addressing, and other protocol resources. This includes the registration of commonly used port numbers for well-known internet services.

The port numbers are divided into three ranges: the well-known ports, the registered ports, and the dynamic or private ports.

The well-known ports (also known as system ports) are those numbered from 0 through 1023. The requirements for new assignments in this range are stricter than for other registrations.