Description: In active mode FTP the client connects from a random unprivileged port (N > 1024) to the FTP server’s command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The server will then connect back to the client’s specified data port from its local data port, which is port 20.
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Java FTPServer.doc