When an inbound network request is made, such as that from a web browser or FTP client, it is sent to the IP address of the server. In addition, the request carries inside it a port number (or just port), which is a 16-bit value placed near the beginning of a network packet. The port number defines the type of server software that should respond to the request. For example, by default, web browsers send requests encoded for port 80. Web servers “listen” to port 80 and respond to incoming requests. The encoded port can be considered part of the address of a request. While the IP address specifies a particular interface (or host), the port specifies a specific service available on that host. Many port numbers are predefined, and the list is expanded as needed to accommodate new technologies. The official list of port number assignments is managed by the Internet Assigned Numbers Authority (IANA). The ports known by your system are listed in /etc/services.

Port numbers 1 through 1023 are often referred to as privileged ports because the services that use them often run with superuser authority. Many of these, such as ports used for FTP (21), Telnet (23), and HTTP (80), are often referred to as well-known ports because they are standards. Port numbers from 1024 through 65535 (the maximum) are unprivileged ports and can be used by applications run by ordinary system users.

During the initial contact, the client includes a local (randomly selected) unprivileged port on the client ...

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