For TCP and UDP, 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, a
network address such as 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 TCP and UDP 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. The registered ports are those from 1024 through 49151. IANA maintains the official list of well-known and registered ranges. The dynamic or private ports are those from 49152 through 65535. One common use for this range is for
ephemeral ports. ==Network behavior==