Telnet: Connecting the Dots. Why it still matters today?

Every time I connect to a remote server with Telnet, I'm reminded how this vintage yet powerful tool unlocks a complex dance between IPv4 and IPv6 addresses that keeps the digital world connected. Software engineers and network administrators have used Telnet to peek behind the scenes of computers far away.

Telnet works like a simple phone call over the internet to a remote computer. By typing a straightforward command like

         telnet <ip address of remote computer> <port number>

I can connect to that computer and check which ports are open and listening. 

IP addresses come in two flavors: IPv4 and IPv6. Think of IPv4 as a classic phone line and IPv6 as a modern digital line. When I use a hostname instead of an IP address, Telnet asks the Internet's directory (DNS) for both IPv4 and IPv6 addresses. Telnet usually tries to connect to modern IPv6 first. If it is not reachable or the line is busy, it falls back to the trusty IPv4 classic line. No complicated setups needed. Telnet will handle this smoothly behind the scenes.

For software testers, this is extremely handy. Because Telnet can quickly confirm if

  •     An application's backend services are reachable
  •     A database port is open
  •     A web server is accessible over a specific port
When something is not working, it helps to pinpoint whether the issue lies with the network, the firewall, or the service itself. 

Suppose when I am trying to connect to a remote mail server on port 25 (SMTP) using the command: 

          telnet mail.example.com 25

  1. If the connection is successful and I see the response, then the server is reachable, the service is on port 25 and is listening and running, and no firewall is blocking my connection.
  2. If I get the response 'Connection refused', it means that the server exists, but the SMTP service is not running, or the firewall is blocking the connection to port 25. Here, the server responded to my request but refused to connect.
  3. If the connection simply keeps waiting but nothing happens, it means either my computer can not reach the server or a firewall is blocking me silently. Here, the server might not exist, or something is stopping my connection without responding with any message to me.
In Windows, 'Telnet Client' is turned off by default. To enable it easily, go to Control Panel, open 'Programs and features' and click on 'Turn Windows features on or off'. Check the box next to 'Telnet Client' and click OK. Windows will install the necessary files, and after completion, Telnet will be ready to use. This is the simplest method for obtaining Telnet access for testing and network troubleshooting purposes.

Think of Telnet as a basic but powerful diagnostic stethoscope for the network. Despite newer tools in the market taking over many of its roles and Telnet having many security concerns, it remains an efficient option for quick connectivity checks and simple text-based communication. It reminds me that sometimes, the simplest tools still make the biggest difference in day-to-day testing work.

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