What is port 20 Used For?

Port 20 is traditionally associated with FTP (File Transfer Protocol) data transfer. FTP, which was established in the early days of the internet, is a protocol that computers use to transfer files between one another.

FTP and Port 20

FTP operates on a client-server model. The client initiates a connection to the server, requesting file downloads or uploads. In this scheme, FTP uses two separate connections - one for commands and another for data transfer. The command connection, usually occurring on Port 21, is employed for control messages exchanged between the client and the server, including authentication, commands, and responses. The data connection, which traditionally uses Port 20, is where actual file data is transferred. This method of separating command and data connections is also known as FTP's out-of-band control scheme.

Active FTP vs. Passive FTP

FTP's utilization of Port 20 varies based on whether it is using an "active" or a "passive" mode for data transfer. In active FTP, the client opens a port for incoming data from the server. The server then initiates a data connection from its Port 20 to the client's specified data port. On the other hand, in passive FTP, the server opens a port (not necessarily Port 20) and awaits an incoming data connection from the client. This mode is generally preferred for navigating through firewalls since the client initiates all connections.
Although Port 20 has been traditionally associated with FTP data connections, its use isn't limited to FTP. Like other ports, it can be used by any application to send and receive data over a network.