When communicating over FTP, two ports are used, one for commands and the other for data.
Here’s how to calculate the ftp data port:
227 Entering Passive Mode (10,10,1,11,19,15)
10,10,1,11 is the server’s TCP/IP address.
19,15 is the port that the server is telling the client to use during the data transfer. To calculate this port, use the formula: (first value x 256) + second value.
This example has a value of (19 x 256) + 15 = 4879.
So the data port for this FTP connection will be port number4879.
See also: RFC 959 – File Transfer Protocol