What is port 23 used for?
Port 23 is Telnet, a remote text terminal: whatever you type is sent to the remote device and its output comes back. It was the standard way to manage servers, switches and routers before SSH.
Telnet has no encryption at all. Anyone who can capture the traffic reads the user name, the password and every command. For that reason it is disabled on current systems and replaced by SSH, but it survives on old network gear, industrial equipment and embedded devices, where finding it open is a finding in itself. The telnet client program is also still used as a quick test: pointing it at another port, such as 25 or 80, shows whether that port answers.
Is port 23 TCP or UDP?
TCP. Telnet is a stream of characters that must arrive in order, so it runs over TCP on port 23.
What is the secure alternative to Telnet on port 23?
SSH on port 22. It offers the same remote command line but encrypts the session and verifies the server's identity. The fix for an open port 23 is to enable SSH, then disable the Telnet service.
Port 23 vs port 22: what is the difference?
Port 23 (Telnet) is clear text; port 22 (SSH) is encrypted. They do the same job, so there is no reason to keep 23 open on a device that supports SSH. Port 22 explained
Which CompTIA exams list port 23?
CompTIA lists port 23 by number in both the Network+ and the A+ Core 1 objectives.
- Network+ N10-009: objective 1.4 (Networking Concepts), listed as “Telnet, 23”.
- A+ Core 1 220-1201: objective 2.1 (Networking), listed as “23, Telnet”.
The quoted text is how the objectives document lists the port. The exam puts the port in a scenario, such as a firewall rule or a failed connection; it does not only ask for the number.
Check yourself: a practice question
Written by CertSprint for this page. It is not taken from any exam.
A scan of a factory network finds TCP 23 open on several controllers. Why is this reported as a risk?
- Telnet sends credentials and commands unencrypted
- Port 23 is reserved for malware
- Telnet uses too much bandwidth
- Port 23 blocks DHCP
Show the answer
A. Telnet sends credentials and commands unencrypted. Anyone who can capture traffic on the path can read the sign-in and every command. The remedy is SSH, or isolating the devices that cannot support it.
Sources: IANA Service Name and Transport Protocol Port Number Registry: port 23; RFC 854: Telnet Protocol Specification. The port number and how each exam lists it come from CompTIA's exam objectives, linked from the study plans above. The explanation and the question are CertSprint's own, checked against these sources on 12 November 2026.