CompTIA A+ Networking Fundamentals: Ports, Protocols, and Troubleshooting

Why Networking Fundamentals Matter on the CompTIA A+ Exam

If you’re preparing for the CompTIA A+ certification, CompTIA A+ Networking Fundamentals is one of the most important topic areas you’ll encounter. The Networking domain accounts for 20% of the Core 1 exam (220-1201) — that’s a significant chunk of the up-to-90 questions you’ll face in your 90 minutes. You need to pass Core 1 with a score of at least 675 out of 900, and a solid grasp of networking concepts will go a long way toward getting you there. Let’s break this down so you know exactly what to study and how to approach it on exam day.

Key Ports and Protocols You Must Know

The CompTIA A+ exam expects you to know specific port numbers and what services use them. This isn’t about memorizing a list — it’s about understanding why each protocol exists so you can apply that knowledge in troubleshooting scenarios. Here are the ports that appear most frequently:

• FTP (20/21) — File Transfer Protocol. Port 20 carries data, port 21 handles control commands. Used for transferring files between systems, but transmits data in plaintext.
• SSH (22) — Secure Shell. Provides encrypted remote access to systems. Also used for SFTP, which is the secure alternative to FTP.
• Telnet (23) — An older remote access protocol that sends data unencrypted. The exam often contrasts Telnet with SSH to test your understanding of security trade-offs.
• SMTP (25) — Simple Mail Transfer Protocol. Handles outgoing email between mail servers.
• DNS (53) — Domain Name System. Translates human-readable domain names (like certcy.app) into IP addresses. If DNS fails, users can’t reach websites by name — a common troubleshooting scenario.
• DHCP (67/68) — Dynamic Host Configuration Protocol. Port 67 is used by the server, port 68 by the client. DHCP automatically assigns IP addresses, subnet masks, default gateways, and DNS server information to devices on a network.
• HTTP / HTTPS (80 / 443) — HyperText Transfer Protocol and its secure variant. Port 80 sends data unencrypted; port 443 uses TLS/SSL to encrypt web traffic.
• POP3 (110) / IMAP (143) — Both are used to retrieve email. POP3 downloads and typically removes email from the server; IMAP syncs email across multiple devices.
• SMB (445) — Server Message Block. Enables file and printer sharing across Windows networks.
• RDP (3389) — Remote Desktop Protocol. Allows graphical remote access to Windows systems. A key port to know for troubleshooting remote connectivity issues.

TCP/IP and Subnetting Basics

Understanding how IP addressing works is foundational to networking troubleshooting. The CompTIA A+ exam tests you on CIDR (Classless Inter-Domain Routing) notation, which is how subnet masks are expressed as a prefix length.

For example, a /24 subnet mask means the first 24 bits of an IP address identify the network, leaving 8 bits for host addresses — giving you up to 254 usable hosts. This corresponds to the subnet mask 255.255.255.0, which is by far the most common in small business and home network environments. When you see an IP address written as 192.168.1.0/24, you immediately know the network can hold up to 254 devices.

A /16 subnet (255.255.0.0) provides 16 bits for hosts — over 65,000 addresses — while a /8 subnet (255.0.0.0) is used for very large networks. The exam may present a scenario where a device has an APIPA address (169.254.x.x), which tells you DHCP has failed and the device assigned itself an address — a critical clue in troubleshooting.

Network Hardware and Protocols That Support the Infrastructure

Beyond ports and IP addressing, the exam tests your understanding of the protocols and devices that keep networks running:

Switching and the Spanning Tree Protocol (STP)

In networks with redundant switch connections (which are common for reliability), there’s a risk of broadcast loops — a scenario where a packet bounces endlessly between switches, consuming all available bandwidth. The Spanning Tree Protocol (STP) prevents this by automatically disabling redundant paths and only activating them if the primary path fails. If you see a question about why a network slows down catastrophically with redundant switches, STP is your answer.

DHCP vs. Static IP Addressing

DHCP dynamically assigns network configuration to devices — IP address, subnet mask, default gateway, and DNS server — so administrators don’t have to configure each device manually. In contrast, servers, printers, and network infrastructure devices typically use static IP addresses so their address never changes. On the exam, if a user’s device shows an APIPA address (169.254.x.x), that’s your signal to check whether the DHCP server is reachable.

DNS Resolution

DNS is what allows you to type