FTP usually uses two well known port numbers. These numbers can be (and often are) changed by the admin for security reasons. Also depending on the mode the the data port is not always port 20. 20 FTP -- Data 21 FTP -- Control

FTP runs in two modes active and passive and FTP uses TCP exclusively

In active mode the FTP client connects from a random port(any port above 1023 we will call it X) to the FTP servers control port, port 21. Then the client starts listening on port(X + 1) and sends the FTP command [port "X+1"] to the FTP server. The FTP server will then connect back to the client's specified data port(port X+1) from its local data port(port 20).

In passive mode the client initiates both connections to the server. To open the FTP connection the client opens two ports locally. X & X+1. The client's first port contacts the server on it's control port, port 21. Next instead of of sending the PORT command the client sends the PASV command. The server then opens a random port sends back a P in response to the client. Then the client initiates the connection from X+1 to P on the sever to transfer data. Thus the data port on the server is random and not port 20. These are just a few of the RFC's regarding FTP RFC 114<originally FTP runs on NCP RFC 765<replaced by a TCP/IP version RFC 959<adds a bunch of new optional commands and is the standard for today. RFC 2228<adds security extensions RFC 2428<adds support for IPv6 RFC 5797<command and extension registry