What is Telnet and how does it work?

Many older technologies are no longer widely used but are still important to understand. Telnet is one of these technologies. It was once the main way to access and manage computers remotely over a network. 

Today, it has mostly been replaced by safer options, but learning how Telnet works, its history, and its security problems helps us understand how network security and remote access have improved. This guide explains what Telnet is, how it works, why modern tools like SSH replaced it, and more.

What is Telnet?

Telnet, an acronym for Teletype Network (or Telecommunications Network in earlier contexts), is a client-server application protocol that facilitates a text-based, bidirectional communication channel between two machines over a network. 

It essentially allows a user on a local computer (the client) to log in and execute commands on a remote computer (the server) as if they were physically present at that remote system's console.

Modern administrators often move beyond simple terminal access to better network designs to ensure scalable remote management

The original purpose and brief history of Telnet

The Telnet protocol emerged in the late 1960s as a foundational component of the Advanced Research Projects Agency Network (ARPANET), the precursor to the modern internet. Its original purpose was to provide remote terminal access, enabling users to control distant mainframes and minicomputers. Initially, it was perceived as a simple and effective method for connecting to remote systems, allowing users to interact with them via a command-line interface.

Telnet was formalized through a series of Request For Comments (RFCs), with RFC 854 and RFC 855 standardizing the protocol in 1983, collectively forming Internet Standard 8. 

This early standardization reflects its critical role in establishing remote connectivity in the nascent stages of network computing. Over time, as security threats grew, Telnet's limitations became apparent, leading to its decline in widespread use for sensitive operations.

How does Telnet work?

Telnet enables remote access by creating a direct communication session between a user’s computer and a remote system. Its operation is based on several key components and steps.

Client–server model

Telnet follows a client–server architecture. The user’s device runs a Telnet client, which connects to a Telnet server running on the remote machine. Once connected, the user can interact with the remote system as if they were physically present at it.

Network Virtual Terminal (NVT)

To allow different operating systems to communicate smoothly, Telnet uses a standard format called the Network Virtual Terminal (NVT). The client converts user input into this universal format before sending it, ensuring compatibility between diverse systems.

TCP/IP connection (Port 23)

Telnet establishes a reliable connection using TCP/IP, typically over port 23. This connection enables two-way communication, allowing commands to be sent to the remote system and responses to be returned in real time.

Plain text communication

All Telnet data, including usernames, passwords, and commands, is transmitted in plain, unencrypted text. This lack of encryption makes Telnet vulnerable to interception and is a major reason it has been replaced by more secure protocols.

While Telnet focuses on command execution, organizations frequently use network file systems to manage how data is stored and accessed across these remote connections.

What are Telnet's major security risks?

Telnet was designed in an era when network security was not a primary concern. As a result, it lacks essential protections, making it highly vulnerable to modern cyber threats.

Clear-text transmission

Telnet sends all data, including usernames, passwords, and commands, in unencrypted plain text. Anyone on the same network can intercept this traffic using packet-sniffing tools (such as Wireshark) and read the information instantly.

Credential theft & session Hijacking

Because login details are exposed, attackers can steal credentials and take over active sessions. This allows them to impersonate legitimate users and gain full control of the remote system.

Man-in-the-Middle (MITM) attacks

Telnet does not verify the integrity or authenticity of communications. Attackers can intercept traffic between the client and server, read or alter the data, and even inject malicious commands without detection.

No built-in authentication or encryption

Telnet lacks modern security features such as encrypted connections and public/private key authentication. This makes it vulnerable to brute-force attacks, where attackers repeatedly attempt to guess login credentials.

Easy exploitation

Due to these weaknesses, Telnet services are frequent targets for attackers. Compromised systems can be used to gain root access, deploy malware, or recruit devices into botnets.

Telnet vs. SSH

Telnet is simple but insecure, making it unsuitable for modern networks. SSH, on the other hand, provides encrypted and authenticated communication, making it the preferred choice for secure remote access.

What are the practical use cases for Telnet in modern networks?

Despite its widespread deprecation for security-sensitive tasks, Telnet retains some niche applications in modern networking environments, primarily due to its simplicity and low overhead.

1. Testing network connectivity

Telnet can be used to check whether a specific port on a remote server is open and reachable. Network administrators often use it to verify service availability (e.g., testing SMTP on port 25 or HTTP on port 80).

Example use: Confirming whether a mail server is accepting connections.

2. Troubleshooting network services

Because Telnet allows manual interaction with services, it is useful for diagnosing protocol issues. Administrators can send raw commands to services like web servers or mail servers to observe responses.

Example use: Sending manual SMTP commands to debug email delivery problems.

3. Managing legacy systems

Some older hardware and software, such as legacy routers, switches, or industrial control systems, only support Telnet for remote access. In isolated or secure internal networks, Telnet may still be used to manage these systems.

Example use: Accessing outdated network equipment that lacks SSH support.

4. Internal lab and training environments

Telnet is sometimes used in educational settings and lab environments to demonstrate networking concepts such as client–server communication, port usage, and protocol behavior without the complexity of encryption.

Example use: Teaching students how remote connections and command execution work.

5. Quick, lightweight remote access in controlled networks

In tightly controlled, air-gapped, or trusted networks where security risks are minimal, Telnet may be used for quick access due to its simplicity and low resource usage.

Example use: Accessing devices within a closed testing environment.

For proactive oversight beyond manual testing, specialized network monitoring tools provide real-time visibility into port health and device availability.

How to use the Telnet client?

While Telnet's usage is discouraged for sensitive data, knowing how to use its client can be beneficial for specific troubleshooting or legacy access scenarios.

Enabling and using Telnet in Windows

The Telnet client is often included but disabled by default in modern Windows operating systems.

To enable it:

1. Go to Control Panel > Programs > Programs and Features.

2. Click Turn Windows features on or off.

3. In the Windows Features dialog box, check the box next to Telnet Client.

4. Click OK and wait for the installation to complete.

To use it via Command Prompt:

1. Open the Command Prompt (search for cmd in the Start menu).

2. Type telnet [IP address or hostname] [port number] and press Enter.

• For example: telnet 192.168.1.1 23 to connect to a router, or telnet google.com 80 to test HTTP connectivity.

1. If successful, you'll see a blank screen or a welcome message from the remote server, awaiting your commands.

Accessing Telnet on macOS and Linux

On macOS and Linux, the Telnet client is typically pre-installed or easily installable via package managers.

To use it via Terminal:

1. Open the Terminal application (found in Applications/Utilities on macOS, or via your distribution's application menu on Linux).

2. Type telnet [IP address or hostname] [port number] and press Enter.

• For example: telnet myserver.local 23 or telnet irc.example.org 6667.

1. The terminal will display output from the remote server, and you can type commands directly.

Essential Telnet commands and their syntax

Within a Telnet session, commands can be categorized into two types: those sent to the remote system and special Telnet protocol commands.

Standard Commands (sent to the remote system):

These are commands specific to the remote operating system or application you are connected to. For example, if connected to a router CLI:

• show ip config: Displays network configuration.

• ping [host]: Sends test packets to a network host.

• help: Lists available commands.

Telnet Protocol Commands:

These are special commands prefixed by the Interpret As Command (IAC) escape character (decimal 255). They are used to negotiate session parameters and control the communication. Some key IAC commands include:

• IAC WILL [option code]: This party offers to enable a specific option or agrees to the other party's request to enable an option.

• IAC WON'T [option code]: This party refuses to enable an option or offers to disable one.

• IAC DO [option code]: This party requests the other party to enable an option or agrees to their offer to enable.

• IAC DON'T [option code]: This party requests the other party to disable an option or agrees to their offer to disable.

Common option codes include 0 for Binary Transmission, 1 for Echo, 3 for Suppress Go Ahead, 24 for Terminal Type, and 32 for Terminal Speed. These allow the client and server to agree on how to handle character transmission and display during the session.

To maintain security when enabling features across multiple workstations, consult our Windows MDM complete guide for automated configuration strategies.

Conclusion

Telnet, an early remote-access protocol, played a key role in the development of internet networking. Its simple, text-based communication made remote system control possible, but its lack of encryption exposed sensitive data to eavesdropping and man-in-the-middle attacks, making it unsuitable for secure use.

Today, SSH has replaced Telnet as the standard for secure remote access, offering strong encryption and authentication. Still, Telnet remains useful in limited, controlled environments, such as managing legacy devices, troubleshooting connectivity, and working with older industrial or IoT systems. Studying Telnet helps us understand both the history of networking and the importance of modern security practices.