An administrator receives a call from an end user reporting that the application they are using keeps giving them an error on port 23. Which of the following is being used?

SSH (Secure Shell) typically uses port 22 for secure remote command-line access, not port 23.

Remote Desktop Connection (RDP) commonly uses port 3389 for graphical remote access, not port 23.

Port 23 is the well-known port number specifically assigned to the Telnet protocol. Telnet is an old, insecure network protocol used to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection.

VPNs use various protocols and ports, such as L2TP/IPsec (UDP 500/4500) or PPTP (TCP 1723), but port 23 is not a primary or exclusive port for VPN services.