Understanding Port 23 and Telnet: A Practical Guide for Modern Networks

Introduction

Port 23 is the traditional gateway for the Telnet protocol, a nearly iconic staple in the history of computer networks. Telnet, short for “teletype network,” enables a text-based, interactive login to remote machines over a TCP connection. For decades, administrators used Telnet on port 23 to manage routers, switches, servers, and various network appliances. Yet the convenience of a straightforward command line hides a set of security challenges that every modern IT team needs to understand. In this article, we explore what port 23 and Telnet are, why they still appear in some environments, and how to approach them responsibly in today’s security-conscious landscape.

What Telnet Is and How Port 23 Fits In

Telnet is a client-server protocol that provides a bidirectional interactive text session over a network. When a Telnet client connects to port 23 on a server or device, the session is established, and keystrokes are transmitted in plaintext. This means credentials, commands, and responses travel without encryption. The reliance on port 23 for Telnet has a long pedigree; many operating systems and network devices expose Telnet services using this port by default. While that default configuration was once convenient, it also creates a predictable target for attackers scanning for open Telnet services on port 23.

Why the default port matters

Consistency: Port 23 is widely recognized as the standard entry point for Telnet, so many scripts and management tools look for it by default.
Visibility: Open port 23 is easy to discover with basic network scanning, making it a common attack surface in poorly secured networks.
Historical baggage: Because Telnet predates modern encryption, relying on this port without protections can expose sensitive information.

Security Concerns with Telnet on Port 23

Security is the central reason many organizations move away from Telnet on port 23. Here are the main concerns to be aware of:

Plaintext credentials: Usernames and passwords travel in clear text, which means anyone with access to the traffic can read them.
Susceptibility to eavesdropping: Even on private networks, unencrypted Telnet traffic can be captured by attackers using simple tools.
Man-in-the-middle risks: Without encryption, attackers can intercept and alter Telnet sessions, potentially issuing commands or harvesting data.
Weak authentication: Some Telnet deployments rely on basic or default credentials, increasing the chance of unauthorized access.
Compliance and governance: Industry standards increasingly require encryption for remote access to systems, often making Telnet on port 23 non-compliant.

Because of these risks, security teams often recommend disabling Telnet on port 23 where possible and replacing it with encrypted alternatives. When Telnet remains in use, it should be isolated behind strong access controls, monitored, and restricted to trusted management networks.

How Telnet Works in Practice

Understanding the mechanics helps explain why the security concerns are so important. Telnet operates over the Transmission Control Protocol (TCP). A client connects to a server on port 23, which establishes a session. The Telnet protocol negotiates options, such as terminal type and line mode, and then forwards keystrokes and textual responses in real time. The result is a responsive, interactive shell that can control a remote device. Because the data is unencrypted, the entire session, including authentication, is visible to anyone who can access the network path.

Key interaction points

Connection establishment: TCP handshake followed by Telnet option negotiation.
Login phase: Credentials are transmitted in plaintext unless additional protections are present.
Command execution: The server interprets text-based commands and returns results in clear text.
Session termination: The user or the system ends the session, closing the TCP connection on port 23.

Real-World Uses and Legacy Scenarios

Despite its drawbacks, Telnet on port 23 still shows up in certain environments. Legacy devices, older network gear, and some embedded systems continue to expose Telnet for practical reasons—especially when the device firmware lacks updates or vendor support for modern protocols. In these cases, Telnet can offer quick, human-readable access for troubleshooting. The challenge is balancing access with security and compliance. For example, a data center might still rely on Telnet to reach a legacy console on a network switch or a console server, where replacing hardware would be expensive or impractical. In such contexts, administrators may implement Telnet in tightly controlled segments, limiting exposure through firewall rules and strictly monitored maintenance windows.

Best Practices: Replacing Telnet with Secure Alternatives

For most organizations, the best practice is a gradual migration away from Telnet on port 23 toward encrypted remote access. The leading alternative is SSH, which provides authentication, encryption, and integrity checks, making remote administration far more secure. Here are practical steps to follow:

Disable Telnet where possible: Remove Telnet services from devices and restrict access to management networks only.
Use SSH as the default remote access protocol: Prefer SSH on port 22 or another secure port, with strong key-based authentication and, if needed, multi-factor authentication.
Segment networks and apply strict access controls: Place devices that must be managed by Telnet behind firewalls and VPNs, and restrict port exposure to a small set of trusted hosts.
Monitor and log Telnet activity: If Telnet must exist temporarily, enable detailed logging and integrate alerts for unusual login attempts or session durations.
Plan a firmware upgrade strategy: Where devices support SSH, update firmware and disable outdated features to reduce risk.
Educate administrators: Provide guidance on secure credentials, locking out after failed attempts, and recognizing suspicious activity.

How to Test and Audit Port 23 Usage

Regular assessments help keep a network secure. If you want to verify whether port 23 is open or in use, consider the following steps:

Network scanning: Use a controlled, compliant scanner to identify devices listening on port 23 within your network.
Service inventory: Maintain an up-to-date catalog of devices offering Telnet services and verify their necessity.
Access controls review: Check firewall rules and ACLs to ensure only authorized hosts can reach port 23.
Credential hygiene: Audit credentials associated with Telnet sessions, and enforce rotation and strong passwords if any Telnet accounts remain.
Migration plan tracking: Document milestones toward SSH-based management and decommissioning Telnet services.

Future Trends and Considerations

The trend in modern networks is clear: encryption and strong authentication are non-negotiable for remote management. As devices evolve, more vendors embed secure management protocols and replace Telnet with SSH or proprietary encrypted channels. In environments that must rely on legacy capabilities, administrators should pursue network segmentation, enhanced monitoring, and rapid migration plans. Port 23 telnet usage is likely to decline over time as security controls tighten and compliance regimes become stricter. Organizations that stay current will reduce risk while maintaining operational efficiency during transitional periods.

Conclusion

Port 23 and Telnet represent a key chapter in the history of network administration. Their simplicity and long-standing presence made them indispensable in the past, but today they carry significant security implications. By understanding how Telnet operates, recognizing the associated risks, and following best practices that favor encrypted alternatives like SSH, IT teams can safeguard remote management while preserving the ability to support legacy devices during a controlled transition. The goal is not to erase the past but to build a secure future where remote access remains reliable, auditable, and resistant to modern threats.

Key Takeaways

Port 23 is the default for Telnet, which transmits data in plaintext and poses security risks.
Telnet remains in use in some legacy environments, but it should be isolated and monitored.
SSH is the preferred secure alternative for remote administration.
Regular testing, auditing, and a clear migration plan are essential for maintaining secure networks.