Port numbers are divided into three types. Well known, registered and dynamic
Well Known Ports
These range from 0 to 1023 and are well known and defined. For the sake of simplicity UDP and TCP port numbers for the same service are the same. Examples are
smtp 25 / tcp Simple Mail Transfer Protocol
smtp 25 / udp Simple Mail Transfer Protocol
http 80 / tcp World Wide Web HTTP
http 80 / udp World Wide Web HTTP
Registered Ports
These range from 1024 through 49151
The full list of port numbers can be accessed from the IANA (Internet Authority of Names and Numbers) website at http://www.iana.org/assignments/port-numbers
Dynamic Ports
These ports range from 49152 through 65535 and are generated by the operating system. A value of 0 in the port numbers registry below indicates that no port has been allocated
The purpose of keeping the list of port numbers handy, especially the well known and registered port numbers is to help you identify the type of application associated with the port number. This proves useful when conducting security audits or viewing logs. A good practice is to memorize the more common ones like DNS, DHCP, HTTP, HTTPS
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