CompTIA A+ Port Numbers Cheat Sheet 2026: 20 Essential Ports You Must Memorize

Struggling to remember which port goes with which protocol? You're not alone. Port numbers are one of the most frequently tested topics on the CompTIA A+ Core 1 (220-1101) exam, yet they're also one of the trickiest to memorize.

The good news? You don't need to memorize all 65,535 possible ports. CompTIA focuses on a core list of 20 essential ports that cover the networking protocols you'll encounter in real IT support roles.

This comprehensive guide provides everything you need: a complete port numbers cheat sheet, memory tricks that actually work, interactive practice scenarios, and troubleshooting tips you can use immediately.

Why Port Numbers Matter for CompTIA A+ (and Your IT Career)

Port numbers are like apartment numbers in a building. IP addresses get you to the right building (computer), but port numbers direct traffic to the specific apartment (application or service) inside.

On the CompTIA A+ exam:

• Networking domain represents 20% of Core 1 (220-1101)
• Port number questions appear in multiple-choice AND performance-based scenarios
• You'll need to identify ports by number, protocol name, and use case
• Troubleshooting questions often hinge on knowing which ports should be open

In real IT support:

• Firewall configuration requires knowing which ports to open/block
• Email troubleshooting demands understanding SMTP, POP3, and IMAP ports
• Remote support relies on RDP, SSH, and Telnet port knowledge
• Network security depends on recognizing normal vs suspicious port activity

The Complete CompTIA A+ Port Numbers Chart

This table includes all 20 ports you must know for the exam, organized by port number for quick reference.

Port Numbers by Protocol Category

Understanding ports by their function helps you remember them better and apply them in troubleshooting scenarios.

Email Protocols (Most Tested)

Real-world scenario: A user complains they can receive emails but cannot send them. Which port should you check first?

• Answer: Port 25 (SMTP) or 587 (SMTP Submission) for outgoing mail

Key difference:

• POP3 (110) downloads emails to your device and typically deletes them from the server (think: one device)
• IMAP (143) keeps emails on the server and syncs across all devices (think: multiple devices)

Remote Access Protocols

Security insight: Telnet (port 23) sends everything in plain text, including passwords. SSH (port 22) encrypts all traffic. Always choose SSH over Telnet in the real world.

Troubleshooting tip: Cannot RDP to a Windows server? Check if port 3389 is open on the firewall and that Remote Desktop is enabled in Windows settings.

File Transfer and Sharing

FTP uses two ports:

• Port 21: Control connection (login, commands like LIST, DELETE)
• Port 20: Data connection (actual file transfer)

Common issue: Windows file sharing not working? Port 445 (SMB) must be open. Also check ports 137-139 for legacy NetBIOS support.

Web and Application Services

Security note: Modern websites use HTTPS (port 443) with SSL/TLS encryption. HTTP (port 80) is considered insecure and browsers now warn users.

DNS is critical: Without port 53 working, you cannot browse websites by name. You could still access sites by IP address (like 8.8.8.8), but not www.google.com.

Memory Tricks That Actually Work

Mnemonics are the secret weapon for memorizing ports. Here are the best memory tricks used by A+ exam passers:

TCP vs UDP: Which Ports Use Which Protocol?

Understanding whether a port uses TCP or UDP helps you remember the port number AND troubleshoot connectivity issues.

TCP (Transmission Control Protocol):

• Reliable, connection-oriented
• Guarantees delivery with error-checking
• Slower but more reliable
• Used when accuracy matters more than speed

UDP (User Datagram Protocol):

• Connectionless, no guaranteed delivery
• Faster but less reliable
• Used when speed matters more than perfect accuracy

Exam tip: If you forget whether a port uses TCP or UDP, ask yourself: "Does this service need guaranteed delivery?"

• File transfer (FTP)? Yes → TCP
• DNS lookup? No, speed matters more → UDP
• Remote desktop (RDP)? Yes, every keystroke must arrive → TCP

Common Port Number Exam Scenarios

CompTIA loves testing ports through real-world troubleshooting scenarios. Here are the patterns you'll see:

Scenario 1: Email Configuration Troubleshooting

Question: A user reports they can receive emails on their smartphone but cannot send emails. The email server uses standard ports. Which port is most likely blocked on the corporate firewall?

Analysis:

• Receiving email = POP3 (110) or IMAP (143) - Working ✓
• Sending email = SMTP (25 or 587) - Not working ✗
• Answer: Port 25 (SMTP) is blocked

Real fix: Many ISPs block port 25 to prevent spam. Modern email uses port 587 (SMTP Submission) with authentication.

Scenario 2: Remote Access Issues

Question: A technician needs to remotely access a Linux server to run command-line administration tasks. Which port should be open on the firewall for secure access?

Analysis:

• Linux server = Not Windows (not RDP)
• Command-line = Terminal access
• Secure = Encrypted (not Telnet)
• Answer: Port 22 (SSH)

Wrong answers to avoid:

• Port 23 (Telnet) - Unencrypted, insecure
• Port 3389 (RDP) - Windows only
• Port 80 (HTTP) - Web traffic, not command-line

Scenario 3: File Sharing Problems

Question: Users in an office cannot access a shared network drive on a Windows Server. The server is online and pingable. Which port should the network administrator verify is open?

Analysis:

• Windows file sharing = SMB protocol
• Server is online (ping works) = Not a connectivity issue
• Cannot access shares = Likely firewall/port issue
• Answer: Port 445 (SMB)

Also check: Ports 137-139 for legacy NetBIOS support if older Windows clients are involved.

Scenario 4: Web Services Troubleshooting

Question: Users report they can access HTTP websites but receive security warnings when accessing HTTPS websites. Which port might be blocked or filtered?

Analysis:

• HTTP works = Port 80 is open ✓
• HTTPS has problems = Port 443 issue ✗
• Security warnings = SSL/TLS certificate issues OR port blocking
• Answer: Port 443 (HTTPS) is being blocked or filtered

Additional issue: Could also be SSL certificate problems, but port blocking is most common cause.

Study Timeline for Mastering Port Numbers

Most students need 3-5 days of focused study to memorize all 20 essential ports. Here's a proven study plan:

Interactive Practice Checklist

Use this checklist to track your port number mastery. Check off each item as you achieve it.

Port Security and Firewall Considerations

Understanding port security is crucial for both the exam and real IT work.

Common ports to block for security:

• Port 23 (Telnet) - Use SSH instead
• Port 445 (SMB) - Block from internet, allow only internal network
• Port 3389 (RDP) - Never expose directly to internet, use VPN

Ports typically left open:

• Port 80 (HTTP) and 443 (HTTPS) - For web servers
• Port 25 (SMTP) - For mail servers
• Port 53 (DNS) - For DNS servers

Port Scanning Tools for Practice

Get hands-on experience with these tools (legal only on your own networks):

Command-Line Tools:

• netstat -an (Windows/Linux) - View open ports on your machine
• nmap - Scan networks for open ports (advanced)
• telnet [ip] [port] - Test if a specific port is open

Windows built-in:

netstat -an | findstr LISTENING

Shows all ports your computer is listening on.

Linux/macOS:

netstat -tuln

Shows TCP and UDP listening ports.

Frequently Asked Questions

Additional Study Resources

Official CompTIA Resources:

• CompTIA A+ 220-1101 Exam Objectives (free download from CompTIA.org)
• CompTIA CertMaster Practice for A+ (official practice questions)

Hands-On Practice:

• Set up a home lab with VirtualBox
• Practice using netstat commands on your computer
• Configure email clients with different port combinations
• Set up firewall rules to block/allow specific ports

Port Number Practice Quizzes:

• Take daily 5-minute port quizzes until you score 100% consistently
• Use spaced repetition: Review ports you miss more frequently
• Practice both directions: Port → Protocol and Protocol → Port

Final Exam Tips for Port Number Questions

On exam day:

1. Use the whiteboard: As soon as the exam starts, write down all 20 port numbers on your provided whiteboard/notepad before you forget them

2. Read carefully: "Which port does SMTP use?" vs "Which port might be blocked if email cannot be sent?" - Same answer (25), different questions

3. Eliminate wrong answers: If you are not sure, eliminate protocols that don't make sense. Remote desktop on Linux? Not RDP (3389).

4. Think security: CompTIA favors secure protocols. SSH over Telnet, HTTPS over HTTP, IMAPS over POP3.

5. Watch for tricks: Port 80 vs 8080 (both web, but 8080 is alternate). Port 22 vs 23 (one number apart, completely different security).

Summary: Your Port Numbers Quick Reference

Must-know ports (memorize first):

• 22 (SSH), 23 (Telnet), 25 (SMTP), 53 (DNS), 80 (HTTP), 110 (POP3), 143 (IMAP), 443 (HTTPS), 3389 (RDP)

Email ports (heavily tested):

• 25 (SMTP - outgoing), 110 (POP3 - download), 143 (IMAP - sync)

Web ports:

• 80 (HTTP - unsecure), 443 (HTTPS - secure)

File transfer:

• 20/21 (FTP), 445 (SMB)

Remote access:

• 22 (SSH - secure), 23 (Telnet - insecure), 3389 (RDP - Windows)

Port numbers may seem intimidating, but with the right mnemonics, focused practice, and understanding of what each protocol does, you will master them. Most students report that port numbers become one of their strongest exam topics after dedicated study.

Use the interactive elements in this guide, create your flashcards, practice daily, and you will walk into the CompTIA A+ exam confident and ready to ace every port number question.

Ready to test your knowledge? Head over to our CompTIA A+ practice exams to drill port numbers in realistic exam scenarios.