You need to understand Transport Control Protocol (TCP). Ports are a parameter of the TCP. There are many different services that can run using TCP as the mechanism to get data from one place to another. TCP keeps these services separate from each other by assigning a unique "port" to each service. Since the "port" parameter is a 16-bit field, there are 65,536 possible "ports" or services. Some of these, such as port 0, are reserved and not used. The ports from 0-1,023 are called "Well Known Ports." The Well Known Ports are assigned by the Internet Assigned Numbers Authority (IANA) and on most systems can only be used by system (or root) processes or by programs executed by privileged users. The ports from 1,024-49,151 are called "Registered Ports." The Registered Ports are listed by the IANA and on most systems can be used by ordinary user processes or programs executed by ordinary users. They are not controlled by IANA and can be used for most any purpose. However, IANA does maintain a "registry" of port numbers and their common uses as a convenience to the community. The ports from 49,152 through 65,535 are called dynamic or "private ports." As the name implies, these ports are not registered and may be used for anything by any program. A listing of the Well-Known and Registered ports, along with their common uses can be found at http://www.iana.org/assignments/port-numbers. With regard to firewall security, your default policy should be to close all ports except those that need to be open for operational reasons. How you define which ports are open and which are closed will depend on which firewall you are using.
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