Each computer system has 2-16 back doors (or ports) that other computer systems and programs can access. These port numbers are divided into three ranges: the Well Known Ports, the Registered Ports, and the Private Ports.
The Well Known Ports are those from 0 through 1023, and are the ports that other computers will most often connect to if you are running any services. For example if you are hosting a web site from your system it would probably be available to other users on port 80.
The Registered Ports are those from 1024 through 49151, and these ports can be used by normal user applications, processes, and programs. For example if you open a web browser or a mail client your outgoing connection to the server will probably orginate from one of these ports.
The Private Ports are those from 49152 through 65535.
The Well Known Ports are assigned by the IANA (Internet Assigned Numbers Authority) and on many systems can only be used by processes, services, or programs executed by privileged users.
These Ports are used in RFC793 to name the ends of logical connections which carry long term conversations. For the purpose of providing services to unknown callers, a service contact port is defined. This list specifies the port used by the server process as its contact port. These are the ports most frequently targeted in security compromise situations, so it is a good idea to be aware of these ports and what you may have running on them – the less of these doors that you have open, the more secure your system will be.
Useful information about ports can be found at the Internet Storm Center. In the upper left, enter the port number you are interested in.
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