Learn the Purpose of the TCP Port 21 and How it Works With FTP

File Transfer Protocol Uses Port 20 and 21

Documents getting exchanged from files
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The File Transfer Protocol (FTP) provides a means to transfer information online, much like Hypertext Transfer Protocol (HTTP) does through a web browser. FTP, however, operates on two different Transmission Control Protocol (TCP) ports: 20 and 21. Both these ports must be open on the network for successful FTP transfers.

After the correct FTP username and password is entered through FTP client software, the FTP server software opens port 21, which is sometimes called the command or control port, by default.

Then, the client makes another connection to the server over port 20 so that the actual file transfers can take place.

The default port for sending commands and files over FTP can be changed, but the standard exists so client/software programs, routers, and firewalls can all agree on the same ports to make configuration much easier.

How to Connect Over FTP Port 21

If FTP is not working, the correct ports may not be open on the network. This can take place 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.

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

Tip: You can use Port Checker to scan your network to see if the router has port 21 open.

There's also a feature called passive mode that can be used if there are problems with port access behind a router.

In addition to ensuring port 21 is open on both sides of the communication channel, port 20 should also 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.

Once it is connected to the FTP server, the client software prompts with the login credentials—username and password—that are necessary to access that particular server.

FileZilla and WinSCP are two popular FTP clients.