Port 23 — TCP
TCP port 23 is associated with Telnet remote terminal sessions. Telnet provides no native cryptographic protection, so its use should be exceptional and restricted to controlled legacy environments where secure alternatives such as SSH are unavailable.
What is port 23 used for?
Telnet is still found on some older devices, network consoles and embedded systems. It does not encrypt the session by default, so credentials, commands and data may be readable along the network path.
Treat Telnet as a legacy compatibility requirement rather than the default choice for administering modern equipment.
Transport protocol
TCP
Security precautions
Disable Telnet where possible: credentials and data are transmitted without native session encryption.
Should we open this port in the firewall?
Avoid Internet exposure and restrict Telnet to the necessary administration segments. Prefer SSH when supported. If legacy equipment requires Telnet, use segmentation, source filtering and controlled access through a VPN or bastion host.
How do I check this port?
First check that the destination service is listening. On Windows, Test-NetConnection SERVER -Port 23 checks TCP connectivity. On Linux, nc -vz SERVER 23 provides a similar connectivity check.
- Confirm that the service is listening on the device.
- Test the TCP path from the intended administration network.
- If a session opens and then closes, check authentication and service restrictions.
- Review administration ACLs rather than adding a broad Internet rule.
FAQ — port 23 / Telnet
Does Telnet encrypt passwords?
Telnet provides no native session encryption. This is a key reason to prefer SSH.
Does an open port 23 mean Telnet is usable?
It confirms only that a TCP connection can be established. The service may still reject the session because of authentication requirements or ACLs.
When should Telnet be retained?
Mainly when legacy equipment has no safer administration protocol. Isolate and tightly restrict the traffic.