21
FTP control channel for commands and responses
File Transfer Protocol – Control
What is port 21?
Port 21 is the FTP control channel through which the client sends commands (login, LIST, RETR, STOR) and receives server responses. Actual file data flows through port 20 in active mode or a negotiated ephemeral port in passive mode. FTP is a legacy protocol with no encryption — credentials and data are transmitted as plaintext. Modern deployments replace it with SFTP (over SSH) or FTPS (FTP over TLS). Many firewalls block inbound port 21 from the public internet.
Quick checks
$
nc -zv example.com 21
$
nmap -p 21 example.com
$
telnet example.com 21