Passing the Network+ and CCNA exams demands that you know quite a few port numbers, but they’re important to know when working with real-world networks as well. Before we discuss why, here’s a partial list of important port numbers to know for these certification exams:
Ports 20 and 21 are used by File Transfer Protocol (FTP).
Port 22 is used by Secure Shell (SSH).
Port 23 is used by Telnet.
Port 25 is used by Simple Mail Transfer Protocol (SMTP).
Port 53 is used by DNS.
Port 69 is used by Trivial File Transfer Protocol (TFTP).
Port 80 is used by HTTP.
Port 110 is used by POP3.
Port 143 is used by IMAP.
Port 443 is used by Secure HTTP (HTTPS).
When you’re faced with a list such as this, there’s a temptation to ask yourself something like this: “Besides passing the exam, do I really need to know these port numbers?”
The answer is yes. We do a lot of troubleshooting in this business, and part of becoming an expert troubleshooter is knowing how things work when they’re working correctly. That way, when things do go wrong, you can take a structured approach to fixing them.
Let’s say that you’re in the middle of a relaxed day at your network monitoring console. A consulting company is at your site to install a new firewall. While they’re working, your phone starts going off – users are suddenly unable to send and receive email from outside the office. The first thing to ask yourself in such a situation:
“What has changed since what is now broken was working?”
In this case, a firewall is being installed – but do you know why that would stop email? Do you know that ports 25 (SMTP), 110 (POP3), and 143 (IMAP) must remain unblocked by that firewall? If you do, you already know how to fix the problem.
I can tell you from personal experience that the key to becoming a world-class network troubleshooter is mastering the basics, and that includes knowing common port numbers. And if you’re a great network troubleshooter on top of being CompTIA Network+ and CCNA certified, you’re always going to be in demand — and that’s where you want to be! Alternative To WeTransfer