Port 21 (FTP)
Port 21 is the FTP control channel: the client connects here to log in and send commands, while the files themselves travel over a separate data connection (port 20 in active mode, a random high port in passive mode).
- Service
- FTP
- Protocol
- TCP
- IANA registry
- Officially assigned: ftp (File Transfer Protocol [Control]), TCP/UDP
Check if a port is open on your IP
We only check the address you are visiting from, one TCP port at a time. You cannot enter someone else’s address: this is not a port scanner.
Is it safe to open port 21?
Plain FTP sends the username and password unencrypted, so anyone on the path can read them. Prefer SFTP (over SSH, port 22) or FTPS (FTP over TLS, RFC 4217); if you must keep FTP, never expose it to the internet with real accounts.
Sources
“Port 21 is already in use”: how to fix it
The error means another program is already listening on port 21. Find its process ID (PID), then stop that program or start yours on a different port.
Windows (Command Prompt or PowerShell as administrator)
netstat -ano | findstr :21taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:21 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :21sudo lsof -i :21sudo kill <PID>Replace <PID> with the number from the last column (Windows) or the PID column (macOS, Linux).
Why a port can look closed when your server is running
Most home networks sit behind a router doing NAT: the router owns the public address, and your computer has a private one such as 192.168.1.10. A connection from the internet reaches the router and stops there unless you add a port forwarding rule to your computer.
Many mobile and some home providers go further and use carrier-grade NAT (CGNAT): several customers share one public address, and your router itself gets an address from 100.64.0.0/10. Then no rule on your side can open a port; ask the provider for a public IP or use a tunnel or VPN service.
Even with forwarding in place, the firewall on the computer (Windows Defender Firewall, ufw, firewalld) must allow the port, and the program must listen on all interfaces (0.0.0.0), not only on 127.0.0.1.
UDP cannot be checked this way: a UDP service usually stays silent to packets it does not understand, so “no answer” means nothing. This check uses TCP only.
Frequently asked questions
What is port 21 used for?
Port 21 is the FTP control channel: the client connects here to log in and send commands, while the files themselves travel over a separate data connection (port 20 in active mode, a random high port in passive mode).
Is port 21 TCP or UDP?
Port 21 uses TCP. IANA reserves the number for both TCP and UDP, but in practice only TCP is used.
Is it safe to open port 21?
Plain FTP sends the username and password unencrypted, so anyone on the path can read them. Prefer SFTP (over SSH, port 22) or FTPS (FTP over TLS, RFC 4217); if you must keep FTP, never expose it to the internet with real accounts.
How do I free port 21 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :21 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :21 and stop it with kill <pid>.