Application software developers agree to use well-known port numbers that are issued by the Internet Assigned Numbers Authority (IANA). Any conversation bound for the FTP application uses the standard port numbers 20 and 21. Port 20 is used for the data portion and port 21 is used for control. Conversations that do not involve an application with a well-known port number are assigned port numbers randomly from within a specific range above 1023. Some ports are reserved in both TCP and UDP, but applications might not be written to support them. Port numbers have the following assigned ranges:

• Numbers below 1024 are considered well-known ports numbers.
• Numbers above 1024 are dynamically assigned ports numbers.
• Registered port numbers are those registered for vendor-specific applications. Most of these are above 1024.

End systems use port numbers to select the proper application. The source host dynamically assigns originating source port numbers. These numbers are always greater than 1023.

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