Port 21 — FTP control

Security guidance

Plain FTP is unencrypted and heavily scanned. Replace it with SFTP (port 22) or FTPS.

Test port 21 yourself

From your own machine, against any host (swap in your domain):

macOS / Linux — netcat

nc -vz example.com 21

Any OS — nmap

nmap -p 21 example.com

Windows — PowerShell

Test-NetConnection example.com -Port 21

open = a service answered on port 21; closed = the host replied but nothing is listening; filtered = a firewall silently dropped the probe. These test from your network — our port checker tests from the outside, the view that matters for security.

Other common ports