Introduction
Telnet in Computer Networks is one of the oldest and most important remote access protocols. It enables users to log into another computer and operate it with simple textual commands. Telnet uses TCP/IP model and works on port 23 to provide a client-server connection that is reliable. It conveys information in plain text, making it a little less secure over public networks, yet handy in testing and managing internal networks.
Network administrators commonly use Telnet to scan open ports, test servers, or configure older systems. SSH has replaced Telnet as a means of establishing secure communication. In this blog, we will discuss “What is Telnet?”, how it works, its applications, and how it is relevant in today’s fast-paced world.
Before getting into more details, let us first understand the most asked question i.e., “what is Telnet?”
What is Telnet?
Telnet in computer networks means Telecommunication Network. It is a client/server model and is created to telecommunicate between two computers over a TCP/IP network. It enables users to connect to a remote system and control it through a command-line interface.
In a much simpler explanation, Telnet allows you to type in computer commands into your own computer that are executed on some other computer over the internet or even a local network.
- Port 23 is the default port for Telnet
- Type: Application-layer protocol (TCP-based)
- Data format: in Plain text, which is unencrypted and unsecured
Telnet transmits all forms of data, including usernames and passwords, in plain text, which makes it insecure over unsecured networks. Nevertheless, it is still applicable in testing the network and in carrying out internal administration functions.
Now, we have a good understanding to the question “What is Telnet?” Let us now move to the next section, where we will help you understand its functioning.
How Telnet Works?
Telnet uses a client-server model. The Telnet client is operated on your local machine, and the Telnet server (also referred to as a daemon) is operated on the remote machine.
When connecting to a Telnet server, the subsequent processes take place:
- Connection request: The client establishes a TCP connection with the server on port 23.
- Negotiation: Both systems accept a way of communicating (called option negotiation).
- Authentication: The user supplies a user ID and password.
- Command session: The client makes commands; the server then performs them and returns the result.
- Termination: You leave a session when you type exit or logout.
Telnet uses a Network Virtual Terminal. It is like a translator that makes different systems able to communicate with each other.
NVT normalizes control keys or encoding format, as each computer or terminal has its own keys. The telnet client will encode your keystroke to NVT format and communicate it, and the server will decode it to local format. This makes Telnet multipurpose and able to interconnect all types of systems regardless of their native terminal type.
It is important to note that all data moved between the two devices is in plain ASCII text. That’s why anyone intercepting the traffic can read it. This is one of the major reasons why SSH replaced Telnet. In modern networks, SSH is mainly used for secure remote access.
Common Telnet Commands
After the connection to a Telnet server, you may use typical commands that are relevant to the system you have connected to. However, there are also its own commands that are supported by the Telnet client even before that.
First, open your command prompt or terminal and enter the following
telnet
You’ll see a Telnet prompt like:
telnet>
Here are some useful commands:
| Command | Description |
| open <hostname> [port] | Connects to a host (port 23 by default). |
| close | Ends the current session. |
| quit | Exits Telnet. |
| status | Shows current connection status. |
| ? | Displays available commands. |
How to Install and Use Telnet?
The Telnet clients are available on almost all the platforms, but many of them keep it disabled by default due to security reasons and to prevent critical data breach.
On Windows
Open Command Prompt as Administrator and type the given command.
dism /online /Enable-Feature /FeatureName:TelnetClient
Once installed, test it by typing telnet in command prompt.
On Linux based system like Ubuntu and Debian
sudo apt update
sudo apt install telnet
Then you can use it like:
telnet <hostname> [port]
On macOS
However, Telnet was removed from macOS systems like High Sierra and later editions. But you can still install it via Homebrew and other 3rd party methods.
Example using simple administrative commands on terminal.
brew install telnet
Below, we have discussed some Practical Examples on how you can use Telnet in computer networks.
These commands work on Linux, Windows (with Telnet enabled), or macOS (with Homebrew Telnet) installed on them.
- Test an HTTP Connection
To check if a web server is responding on port 80:
telnet example.com 80
If the connection is successful, type:
GET / HTTP/1.1
Host: example.com
(Press Enter twice.)
You’ll see the HTTP response headers and some HTML output. If you get “Connection refused” or “Could not open connection,” the port is closed or blocked.
- Test an SMTP (Mail) Server
Telnet is great for checking mail servers. For example:
telnet smtp.gmail.com 25
If the connection succeeds, try these commands:
HELO test.com
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Telnet Test
This is a test email using Telnet.
QUIT
This helps verify that your mail server is accepting connections and responding properly.
- Check a Custom TCP Port
To check whether a particular port is open, such as 443 to use HTTPS:
telnet example.com 443
When the screen is cleared or there is a blinking cursor, the connection is open.
When it writes Connection refused, the port is blocked or filtered.
- Local Test
You can also connect to your own machine to test the Telnet service:
telnet localhost 23
If the Telnet server is running locally, it will prompt for a login.
Let us now move on to our next section where we will discuss its use cases.
Common Use Cases of Telnet
Even though Telnet is no longer a secure option for remote access, it still has practical uses in specific scenarios.
- Network Troubleshooting: Telnet is one of the quickest tools to test if a port or service is accessible. It’s lightweight and doesn’t require extra setup.
- Legacy Devices: Many older routers, switches, and embedded devices only support Telnet. Admins use it to configure or monitor such systems inside private LANs.
- Learning and Labs: Telnet is simple and easy to understand, making it ideal for networking education. Students can learn about protocols, ports, and client-server communication in a hands-on way.
- Scripting and Automation: In controlled environments, Telnet can be automated using scripts. For example, Python’s telnetlib module lets you log in, send commands, and capture output.
Advantages of Telnet in Computer Networks
Some of the advantages of using Telnet are:
- Simple and lightweight
- Easy to set up
- Works across platforms
- Great for testing network connectivity
- Useful in legacy or closed networks
Limitations of Telnet in Computer Networks
While useful, Telnet comes with serious limitations. Some of these are:
- No encryption: Data is sent in plain text, making it vulnerable to sniffing.
- No authentication verification: It can’t confirm if the server you connect to is genuine.
- No data integrity: Attackers can modify traffic without detection.
- Obsolete in open networks: Most systems now disable Telnet for security reasons.
These limitations led to the creation of SSH (Secure Shell), which provides encryption, authentication, and integrity protection.
Let us discuss the difference between Telnet and SSH.
Telnet vs SSH
| Feature | Telnet | SSH |
| Encryption | None | Encrypted |
| Default Port | 23 | 22 |
| Security | Vulnerable | Secure |
| Authentication | Username/Password | Password or Key-based |
| Use Case | Legacy devices, testing | Remote administration |
| Recommended for | Internal networks | All secure connections |
SSH is the modern, secure replacement for Telnet. It uses cryptographic techniques to protect data and verify connections, making it ideal for production and public networks.
Best Practices if You Still Use Telnet
In some isolated or industrial networks, Telnet may still be needed. Follow these tips to minimize risk:
- Use only in trusted internal networks.
- Restrict access — allow connections only from specific IPs.
- Disable Telnet when not in use.
- Monitor logs for unauthorized access attempts.
- Use a VPN to encrypt traffic if Telnet must cross networks.
- Switch to SSH as soon as your systems support it.
Frequently Asked Questions
Q1. What is Telnet?
Telnet is a network protocol that lets users control computers remotely. It sends text commands over networks but lacks encryption, making data visible to hackers.
Q2. What is the example of Telnet in Computer Networks?
One application of Telnet is remotely accessing a network device, such as a Cisco router or UNIX server, with the command telnet 192.168.1.1 and configuring or administering the device.
Q3. What are two features of Telnet?
Telnet is used to log in to another computer and run commands. Second, it can work across different platforms.
Q4. Does Telnet use HTTP?
No, Telnet does not use HTTP. It runs on TCP port 23 as compared to HTTP, which runs on TCP port 80 (Web communication).
Q5. Does Telnet solely rely on TCP protocol?
Yes, Telnet only relies on the TCP protocol to build stable and connection-oriented communication between the client and server to access remotely and execute commands.
Conclusion
What is Telnet: Telnet is a network protocol for remote command-line access. It uses TCP/IP to establish a remote session. All data, including passwords, is sent in unencrypted cleartext, making it highly insecure. The early development of computer networks heavily relied on Telnet. It laid the foundation for the idea of remote terminal access even before there were secure means of doing so. It is largely no longer used in daily IT operations because it is no longer safe, although it is helpful in learning, troubleshooting, and maintenance of legacy systems.
Knowledge of Telnet in computer networks allows you to understand how remote access evolved between text sessions without encryption and modern guarded, encrypted shells. Although Telnet is no longer used, it is a part of the history of modern networking.
