Understanding TCP/IP

13.3 Passive Mode of FTP Protocol Communication

In some cases, it is possible that a data channel cannot be established from an FTP server to an FTP client (for example, firewalls may deny such a connection). In such cases, we can use the passive mode of communication, where a client creates connections for both the command and data channels. This can be very useful if we want to protect, for example, our network by packet filtration at the access router or a firewall.

In the case of active mode, we cannot use the Telnet program for FTP protocol survey, because we would have to run the Telnet program at the server to create the data channel. In contrast, in the case of the passive mode, we can use the Telnet program instead of the FTP program. ...