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 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 for UDP it is a datagram. Both protocols use a header field for indicating the source and destination port numbers. The port numbers are encoded in the transport protocol packet header, and they can be readily interpreted not only by the sending and receiving hosts but also by other components of the networking infrastructure. In particular, firewalls are commonly configured to differentiate between packets based on their source or destination port numbers. Port forwarding is an example application of this. Port scanning The practice