What Is Port 23 Used For in Networking and Cybersecurity?

In computer networking, ports play a critical role in allowing devices and applications to communicate with each other. Every service running on a network uses a specific port number to send and receive data. One of the most well-known ports in networking history is Port 23, commonly associated with the Telnet protocol.

Although modern enterprises now prefer more secure alternatives, understanding Port 23 is still important for network engineers, cybersecurity professionals, and students preparing for networking certifications. Knowing how it works, why it was used, and the risks associated with it can help professionals better secure modern IT environments.

Let's dig into it.

The Basics: What Exactly Is Port 23?

Port 23 is the default port for Telnet — short for Teletype Network — a protocol developed back in 1969 that allows users to remotely access and manage devices over a network. Think of it as a command-line tunnel between your machine and a remote device like a router, switch, or server.

In its heyday, Telnet was revolutionary. It lets administrators manage hardware from across a building — or across the country — without physically sitting in front of a device. For the era, that was remarkable.

Here's how it typically works:

• A user initiates a Telnet session from a client machine

• The request connects to the target device on Port 23

• A command-line interface opens, allowing the user to execute commands remotely

• The session continues until the user disconnects or the connection times out

Sounds useful, right? It was — until the internet became what it is today.

Why Port 23 Is a Cybersecurity Red Flag

Here's the thing that every cybersecurity professional knows and every beginner needs to learn fast: Telnet transmits data in plain text. No encryption. No masking. Everything — usernames, passwords, commands, responses — travels across the network completely exposed.

In a world where packet sniffers and man-in-the-middle attacks are common tools in an attacker's toolkit, that's a catastrophic design flaw by modern standards. Any threat actor who positions themselves between the client and the remote device can read every single character of that session in real time.

That's why Port 23 and Telnet have been largely replaced by SSH (Secure Shell) on Port 22, which encrypts all traffic end-to-end. But here's the uncomfortable truth: Port 23 is still open on millions of devices worldwide. Poorly configured routers, legacy industrial systems, IoT devices, and overlooked network infrastructure often still run Telnet without anyone realizing it.

From a cybersecurity standpoint, an open Port 23 is essentially an invitation. Threat actors actively scan for it using tools like Shodan and Nmap, and when they find it, they try default credentials. Surprise — many of those attempts succeed.

Real-World Risks Associated with Port 23

The dangers of an exposed Port 23 aren't theoretical. Some of the most significant botnet attacks in history have exploited Telnet:

• The Mirai Botnet (2016) — one of the largest DDoS attacks ever recorded — spread largely by scanning for IoT devices with Port 23 open and logging in with factory-default credentials. The results were devastating, taking down major platforms including Twitter, Netflix, and Reddit.

• Ongoing credential stuffing attacks continue to target network devices with Telnet enabled, particularly in industrial and healthcare environments where legacy systems are common.

• Corporate espionage and data exfiltration have been traced back to undetected Telnet sessions running on overlooked internal network devices.

If your organization still has Port 23 open anywhere on the network, that's not a low-priority item on the to-do list — it's a fire that needs putting out today.

Port 23 in Cybersecurity Certifications — Why It Matters for Your Career

Understanding ports, protocols, and their associated risks is foundational knowledge in cybersecurity — and Port 23 is practically guaranteed to appear on every major certification exam in the field. Here's where it shows up and why studying it properly accelerates your professional growth:

• CompTIA Security+ — covers insecure protocols, port numbers, and their secure alternatives extensively

• CompTIA Network+ — dives deep into port functions, traffic analysis, and network hardening

• CEH (Certified Ethical Hacker) — includes Telnet exploitation as part of real-world penetration testing scenarios

• CISSP — addresses protocol risks at an architectural and policy level

Career opportunities that this knowledge directly supports:

• Network Security Engineer — responsible for auditing open ports and hardening infrastructure

• Penetration Tester / Ethical Hacker — uses port scanning to find vulnerabilities before attackers do

• Security Operations Center (SOC) Analyst — monitors network traffic for suspicious Telnet activity

• Cybersecurity Consultant — advises organizations on legacy protocol risks and remediation strategy

The people who understand why certain ports are dangerous — not just that they are — become the ones organizations trust with their most sensitive systems.

Exam Preparation Tips for Topics Like Port 23

If you're preparing for a certification exam and want to lock in your understanding of ports and protocols, here's what actually works:

• Memorize the common port-protocol pairings — Port 21 (FTP), Port 22 (SSH), Port 23 (Telnet), Port 80 (HTTP), Port 443 (HTTPS). Flashcards work surprisingly well here.

• Understand the why behind security risks, not just the facts. Examiners love scenario-based questions that test reasoning, not recall.

• Practice with Nmap in a lab environment — running port scans on a home network or a virtual machine helps you see how open ports appear to an attacker.

• Take timed practice exams regularly — especially ones that mix protocol and port questions with scenario-based application questions.

What You Should Do About Port 23 Right Now

Whether you're a professional managing a network or a student building your knowledge base, here are the practical takeaways:

• Disable Telnet and close Port 23 on every device under your control — routers, switches, IoT devices, servers

• Replace Telnet with SSH wherever remote access is needed

• Run a port scan on your network using Nmap or a vulnerability scanner to find any open Port 23 instances you didn't know about

• Update default credentials on every network-connected device immediately

• Document and audit your network regularly — open ports have a way of creeping back after firmware updates or configuration resets

The Bottom Line

Port 23 is a perfect case study in how technology ages. What was once a practical innovation is now one of the most exploited vulnerabilities in modern networking. Understanding its history, its risks, and its real-world impact gives you a meaningful edge — both in certification exams and in the field.

The best cybersecurity professionals aren't just people who know the rules. They're the ones who understand the story behind them. And Port 23 has a story worth knowing inside out.