Most TCP ports are general-purpose channels that can be called into service as needed but otherwise sit idle.
Some lower-numbered ports, however, are dedicated to specific applications.
While many TCP ports belong to applications that no longer exist, certain ones are very popular.
Jason Gillikin
TCP Ports 20 and 21
FTP servers use TCPport 21to manage their side of FTP sessions.
The server listens for FTP commands arriving at this port and responds accordingly.
TCP Port 22
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Secure Shell uses port 22.
Jason Gillikin
SSH servers listen on this port for incoming login requests from remote clients.
TCP Port 23
Port 23 governstelnet, a text-based system for logging into remote systems.
TCP Ports 25, 110, and 143
Joli Ballew
Email relies on several standard ports.
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POP3 andIMAPcontrol the flow of emails from your provider’s server to your inbox.
The secure versions of SMTP and IMAP vary depending on configuration, but ports 465 and 587 are common.
Port 443 is the default for secure HTTP.
UDP Port 88
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TheXbox data pipe gaming serviceuses several different port numbers including UDP port 88.
It uses UDP port 162 as the default for receiving SNMP traps from managed devices.
By default, IRC uses port 194.
Joli Ballew
Using registered ports also avoids the additional security restrictions that operating systems place on ports with lower numbers.
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