Port 23 and Telnet: History, Uses, and Security in Modern Networks

Port 23, the well-known TCP port associated with Telnet, stands as one of the oldest yet still relevant elements in the story of network administration. While it once served as the default channel for remote command execution, the security landscape has changed dramatically since Telnet first appeared. This article explains what port 23/tcp represents, how Telnet works, why it is viewed as risky today, and how modern networks manage remote access more securely.

Understanding port 23/tcp and the Telnet protocol

In the TCP/IP model, port numbers help identify specific services running on a host. Port 23 is the default port used by Telnet, a text-based protocol designed for interactive, character-by-character communication with a remote machine. When a Telnet client connects to another device on port 23, it negotiates options and then delivers commands in plaintext. Over time, this simplicity became both a strength and a vulnerability: users could type commands and receive responses in real time, but anyone monitoring the traffic could read every keystroke and password that travels across the network.

Telnet’s architecture is straightforward. A client opens a TCP connection to the server’s port 23, the session begins, and command input is transmitted as plain text. The server executes those commands and returns output back to the client. Because the data stream is unencrypted, the technique is susceptible to eavesdropping, replay attacks, and tampering by attackers who can observe the network path or insert themselves into the conversation. This design trade-off—readability over security—helped Telnet spread quickly in the early days of networking, but it also set the stage for modern password- and data-protection expectations.

A brief history of Telnet and port 23

Telnet emerged in the 1960s and 1970s as a practical solution for remote administration across early networks. It offered a simple interface that users could leverage to manage servers, routers, and other devices from distant locations. The use of port 23 became a convention, codifying how administrators reach out to devices for configuration and troubleshooting. As networking grew, so did the popularity of Telnet in both enterprise environments and educational settings. However, as attackers became more capable, the lack of encryption on Telnet traffic revealed the protocol’s fundamental weakness: sensitive information travels in the clear.

The rise of secure alternatives did not erase Telnet from existence overnight. Some legacy devices, especially older network gear or industrial equipment, still expose Telnet as a management option because it remains familiar to engineers and sometimes required for compatibility reasons. In modern practice, though, the emphasis has shifted toward encrypted remote access and robust authentication to protect critical infrastructure and corporate networks.

Security risks inherent to port 23/tcp and Telnet

The primary risk associated with Telnet is that credentials and command streams are transmitted in plaintext. This makes it relatively easy for anyone who can capture network traffic to read usernames, passwords, and sensitive configuration details. Common exposure scenarios include:

Eavesdropping on unsecured networks, especially in Wi‑Fi, branch offices, or guest networks.
Man-in-the-middle attacks where an attacker intercepts or alters the data between the client and server.
Credential theft through simple packet capture on poorly segmented networks.
Credential reuse or lateral movement after initial access, leveraging weak or default credentials on legacy devices.
Compliance risks for organizations that must protect data and meet industry regulations.

Because of these risks, many security best practices discourage or disable Telnet on devices that can be managed remotely. In environments where Telnet remains for legacy reasons, administrators often take additional precautions to limit exposure, but even with those safeguards, the inherent vulnerability of port 23/tcp remains a concern.

Where Telnet still appears in today’s networks

There are legitimate scenarios where Telnet might be present. In some laboratory or testing environments, Telnet provides a simple, well-understood way to interact with a device. Some older network appliances or industrial controllers may only offer Telnet-based management, especially if they were designed years ago and have not been updated. In tightly controlled networks—air-gapped or physically isolated segments—Telnet can be acceptable if access is restricted and monitored. Nevertheless, in any environment with internet connectivity or mixed networks, port 23/tcp is generally avoided for remote management in favor of more secure alternatives.

Modern alternatives and the recommended security posture

The most widely adopted secure alternative is SSH (Secure Shell), which typically operates on port 22 but can be configured to listen on alternative ports. SSH provides encryption, strong authentication, and integrity protection for remote sessions, making it the preferred choice for remote administration. When possible, organizations should:

Disable Telnet on new devices and migrate to SSH or other encrypted management protocols.
If Telnet must be used for legacy devices, restrict access to a management network or VPN, and place those devices behind strict access controls.
Use network segmentation, ACLs, and firewall rules to limit who can reach port 23/tcp from trusted management networks.
Apply strong authentication, rotate credentials regularly, and monitor login attempts for signs of abuse.
Keep devices updated with security patches and remove any unnecessary services that expose Telnet or similar protocols.

Best practices for administrators managing port 23/tcp traffic

To reduce risk while maintaining the ability to manage legacy gear when necessary, consider the following practical steps:

Audit your network to identify every device exposing port 23/tcp. Create an inventory and a plan to disable Telnet where feasible.
Prefer SSH for remote access and disable Telnet by default on devices that are controllable by administrators.
Implement access controls that limit Telnet access to a dedicated admin VPN or management VLAN with multi-factor authentication where possible.
Enforce encrypted channels for any remote administration, including the use of secure key management and session logging.
Regularly monitor and review Telnet-related logs. Look for unusual login patterns, brute-force attempts, or changes to access rules.
Educate staff and operators about the limitations of Telnet and the importance of secure remote access during change management processes.

How to test and monitor port 23/tcp activity

Administrators should regularly verify the state of port 23 and related services. Basic checks include:

On Linux/Unix systems, use commands such as netstat -tlnp | grep :23 or ss -tlnp | grep :23 to see who is listening on port 23 and which process owns it.
From a network perspective, run a targeted scan with nmap like nmap -sV -p 23 to confirm the service type and version, and to assess exposure.
Review firewall and ACL logs to identify attempts to access port 23 from unauthorized networks.
Test disablement procedures by attempting to connect to Telnet after turning off the feature on devices, ensuring that enforcement is consistent across the fleet.

Conclusion: port 23 in a secure, modern network

Port 23 and the Telnet protocol tell a cautionary tale about the evolution of network security. Telnet’s simplicity made remote management accessible, but its plaintext traffic makes it a poor fit for today’s security expectations. Modern networks prioritize encrypted, authenticated remote access, with SSH and similar protocols taking center stage. While Telnet may still appear in legacy environments, effective security means limiting exposure, replacing or wrapping Telnet with secure channels, and maintaining strict controls around who can access remote devices on port 23/tcp. By understanding the history and risks—and applying disciplined best practices—organizations can keep their networks resilient without compromising on manageability.