TCP Port Number 21 and How It Works With FTP

The File Transfer Protocol provides a framework to transfer information between two networked computers, much like Hypertext Transfer Protocol does through a web browser. FTP, however, operates on two different Transmission Control Protocol ports: 20 and 21. FTP ports 20 and 21 must both be open on the network for successful file transfers.

FTP Port 21 is the Default Control Port

After the correct FTP username and password are entered through FTP client software, the FTP server software opens port 21 by default. This is sometimes called the command or control port by default. Then the client makes another connection to the server over port 20 for file transfers to take place.

The administrator can change the default port for sending commands and files over FTP. However, the standard exists so that client/software programs, routers, and firewalls can agree on the same ports, thus easing configuration.

How to Connect Over FTP Port 21

One reason for FTP to fail is if the correct ports are not open on the network. This blockage may occur on either the server side or the client side. Any software that blocks the ports must be manually changed to open them, including routers and firewalls that might block the ports if the operating system doesn't.

By default, routers and firewalls might not accept connections on port 21. So, if FTP isn't working, it's best to check that the router forwards requests properly on that port and that the firewall isn't blocking port 21.

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.