Which ftp port to use

As we've already mentioned, the default command port for FTP connections is port 21, so, it's important to check if your ISP is not blocking the access to that port.

To check if your router or ISP is blocking the port 21, you should use telnet. If all these tips don't allow you to connect to the server, try to set your FTP client in passive mode. What is the difference between an active and a passive FTP connection and how they work is one of the most asked questions. It is good to have an FTP host which provides support for both, because there are cases when one works and the other refuses a connection. So, what is the difference between the active and the passive mode FTP connections?

The second port is the 'data transfer' port where the real file transfer runs. Typically, the command port is set to port 21 and the data transfer port is port 20, but actually, depending on the connection mode, the data transfer port can be changed. When a file transfer client makes a connection to a port that a file transfer service is listening on, they can then exchange information. Initially, this is in the form of commands.

Commands establish the connection details and operations your want performed. The next step involves transferring the file data requested over the same or a similarly established connection. For this standard case, this is all someone using an FTP client needs to know. More ports are used, with differences based on the type of FTP.

Other ports are also used in the case of SFTP, which is a totally different protocol. FTP has been officially assigned ports 20 and FTP traditionally requires a block of ports to remain open on either the server firewall or the client firewall to aid with the creation of Data Connections.

For security reasons, companies are limiting the number of ports in their publicly facing firewalls and looking for alternate solutions in order to keep ports closed and information secure. Along with file transfers, clients will typically request directory information from the FTP server. This is important because the negotiation of the SSL connection is time-consuming, and having to do it twice—once for the Data Connection and once for the Control Connection—can be expensive if a client plans to transfer a large number of small files.

Therefore, the SSL handshake takes place immediately; it is referred to as Implicit because the port number implies security. Once the server receives this command, the two parties perform an SSL handshake and enter a secure state—hence why port 21 is referred to as Explicit. In addition to ensuring port 21 is open on both sides of the communication channel, port 20 also should be allowed on the network and through the client software.

Neglecting to open both ports prevents the full back-and-forth transfer from being made. When it is connected to the FTP server, the client software prompts with the login credentials—username and password—that are necessary to access that server.

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