The 12 Ports Every A+ Candidate Fails On (And How to Never Forget Them!)

There is no understanding required for port numbers. HTTPS is 443 because a committee said so. That makes them the cheapest marks on CompTIA A+ Core 1 — and, reliably, the ones candidates lose.

The reason is that most people try to memorise a table of forty ports as a flat list. Flat lists of arbitrary numbers are close to the worst thing a human brain can be asked to store. The fix is to cut the list down to what genuinely appears, then learn those in contrasting pairs.

Why ports are free marks you keep giving away

Domain 2.0 Networking is 23% of Core 1. Ports and protocols are the first objective inside it, and they show up again in Core 2 security questions about which services to close. In a 90-question exam you can reasonably expect four to seven questions that hinge on a port number.

Those are marks you either bank in the first ten seconds or lose outright. There is no working it out.

The twelve that actually appear

Learn these before you learn anything else. If you know only this table you will answer the overwhelming majority of port questions correctly.

Port	Protocol	What it is for	Secure twin
20 / 21	FTP	File transfer (data / control)	989 / 990 (FTPS)
22	SSH / SFTP	Encrypted remote shell and file transfer	— already secure
23	Telnet	Remote shell, plain text	22 (SSH)
25	SMTP	Sending mail	587 / 465
53	DNS	Name resolution	853 (DoT)
67 / 68	DHCP	Address assignment (server / client)	—
80	HTTP	Web, unencrypted	443 (HTTPS)
110	POP3	Mail retrieval, download	995
143	IMAP	Mail retrieval, synced	993
389	LDAP	Directory lookup	636 (LDAPS)
443	HTTPS	Web, encrypted	— already secure
3389	RDP	Windows remote desktop	—

Learn them in pairs, never in a list

The single most effective change you can make is to stop reciting "20, 21, 22, 23, 25…" and start learning each insecure port with its secure twin. The exam loves that contrast, because it is the basis of every "which service should you disable" question.

The mail ports are where people come unstuck, because there are three of them and they sound alike. Keep them straight by what they do rather than by number: SMTP sends, POP3 downloads and removes, IMAP syncs and keeps. Once the behaviour is clear the numbers attach themselves.

Verify them yourself in two minutes

Nothing fixes a port number in memory like watching it appear on your own machine. Open a terminal and look at what is actually listening and connected.

See ports in the wild# Windows — every listening port with its owning process
netstat -ano | findstr LISTENING

# Windows — who owns PID 4242?
tasklist /FI "PID eq 4242"

# Linux / macOS — listening TCP and UDP sockets
ss -tulpn

# Prove HTTPS really is 443 — watch the connection open
curl -sv https://corydon-publications.com 2>&1 | grep "Connected to"

# Test whether a specific port is open on a host
Test-NetConnection corydon-publications.com -Port 443   # PowerShell
nc -zv corydon-publications.com 443                    # macOS / Linux

Run those once and the abstractions become concrete. You will see port 443 open as you load a page, watch DNS resolve on 53, and find RDP sitting on 3389 if it is enabled. That five minutes will do more for retention than an hour of staring at a table.

Twelve ports, five pairs, one terminal session. That is the whole job — and it is worth more marks per minute of study than anything else on Core 1.

The four ports people most often confuse

Two clusters cause almost all the wrong answers, and both are worth a moment of deliberate attention. The first is the mail trio — 25, 110 and 143. Candidates blank because the numbers feel interchangeable. Anchor them to behaviour: SMTP on 25 pushes mail outward, POP3 on 110 pulls it down and deletes the server copy, IMAP on 143 keeps everything in sync across devices. Once you think in verbs — send, download, sync — the numbers stop swapping around in your memory.

The second cluster is the remote-access pair: SSH on 22 and RDP on 3389. Both let you control another machine, which is exactly why the exam pits them against each other. SSH is the encrypted text shell for Linux and network gear; RDP is the graphical Windows remote desktop. If a question mentions a graphical Windows session, it is 3389; if it mentions a secure command line, it is 22. Telnet on 23 is the trap answer in both cases — it does the job in plain text, which is why the correct response is nearly always "use SSH instead".

TCP or UDP? A quick sanity check

The exam sometimes asks not just for the number but for the transport protocol underneath. The rule of thumb: if a service needs every byte to arrive in order — web, mail, file transfer, remote shell — it rides on TCP. If it favours speed and can tolerate the odd lost packet — DNS lookups, DHCP, streaming, VoIP — it tends to use UDP. DNS is the classic "both" answer: small queries go over UDP on 53, but large transfers between servers fall back to TCP on the same port. Knowing that one nuance answers a surprising number of questions.

Why the exam tests ports at all

It is fair to ask why a certification would demand rote memorisation of numbers you could look up in seconds on the job. The answer is that ports are a proxy. A technician who instantly knows that HTTPS is 443 and RDP is 3389 has almost certainly worked with those services enough to understand them — the number is shorthand for real familiarity. And in the moments that matter, reading a firewall rule during an incident, or spotting a suspicious open port on a compromised machine, you genuinely do need the number in your head, because you will not have time to search. The exam is using a small, testable fact to check for a larger, harder-to-test competence. Treat the twelve ports not as trivia to survive but as a foundation you will lean on every time you touch a network, and the memorisation stops feeling arbitrary and starts feeling like the first real tool in your kit.

One last piece of advice: write the twelve ports and five pairs from memory every morning for the week before your exam. It takes ninety seconds, it surfaces the ones that have not stuck, and by exam day the list is so automatic that a port question becomes the fastest, most certain mark on the whole paper — a small daily habit that pays off the moment the clock starts.