Port Numbers And The CompTIA A+ Exam

A Comprehensive Overview of Crucial Port Numbers

CompTIA A+ is an entry-level certification generally considered the industry standard for those beginning their career in information technology (IT). Earning this credential is a critical first step, but that doesn’t mean it’s easy. Learners have to gain a fundamental understanding a vast array of technical subjects, including PC hardware, mobile device hardware, and troubleshooting hardware and networking connectivity issues.

One of the core topics that demand significant attention is port numbers, which play a crucial role in computer networking by facilitating communication between different devices and services. A port is a logical endpoint for communication that enables data to flow in and out, ensuring it reaches the proper application or service on a device. Because each port is assigned a unique number -and there are a lot of them - it takes extensive studying to learn what they are used for in the world of networking.

What are Port Numbers?

Port numbers are 16-bit unsigned integers ranging from 0 to 65535. Some (0 to 1024) have preassigned numbers and are called well-known ports. Well-known ports are reserved for privileged services, such as port 80, which is assigned to HTTP (web browsing). Registered Ports (1024-49151) are designated by the Internet Assigned Numbers Authority (IANA) to specific services or applications upon request. Dynamic or Private Ports (49152-65535) are used for dynamic communication between devices.

Port numbers in TCP/IP networking are crucial because they allow multiple applications to use network services simultaneously on a single physical device. They differentiate traffic intended for different applications or services.

TCP vs UDP

Imagine you’re sending a letter to a friend who has two mailboxes, one for important letters (TCP) and another for quick postcards (UDP). Port numbers are like labeling these mailboxes. If you want to send an important letter, you use the "Important Letters" mailbox (TCP port), and if you're sending a postcard, you use the "Quick Postcards" mailbox (UDP port).

TCP (Transmission Control Protocol) is a protocol that rides on top of the IP protocol (Internet Protocol) and is responsible for the connection and continuity of internet communication on any network. To continue the above analogy, think about TCP like sending an important letter that requires confirmation of receipt. When you send a letter, you want to make sure your friend gets it, and if they don't, you'll send it again. In networking, TCP ensures that data (like your letters) reaches its destination correctly. It divides information into smaller packets, sends them, and waits for the receiver to confirm that each packet arrived. If something goes wrong, it'll try to send that part again.

UDP (User Datagram Protocol) is an unreliable, connectionless delivery system for communication that also rides on top of IP. UDP does not establish a virtual connection between the sender and receiver and does not guarantee data delivery. Because it doesn’t have to wait to establish the connection, it is faster than TCP, which is why it is used by applications that require fast and real-time data transmission, such as online gaming and video streaming.

Common Port Numbers and Their Uses

Devices transmit data across a network in packets that hold details on the sender and receiver addresses, as well as the port numbers of both parties. These port numbers represent the specific applications or services sending or receiving the data. A common example is when you use a web browser (like Chrome or Firefox) on your computer, which has to communicate with a web server to load a webpage.

When your computer sends a request to the website’s server, the packet includes the distinct source and destination port numbers. In this specific example, the conventional port numbers are 80 (HTTP traffic)[KC1]   and 443: HTTPS (Hypertext Transfer Protocol Secure). The website’s server then receives the packet and returns a response back to the user’s computer, using the same source and destination numbers.

In real-world scenarios, such as troubleshooting network connectivity issues, understanding port numbers is vital. For instance, diagnosing a non-responsive web server may involve ensuring that the appropriate port (80 or 443) is open and properly configured. Similarly, resolving email delivery issues might require verifying that the SMTP port (25) is not blocked by a firewall. An in-depth knowledge of port numbers is often key to quickly resolving these issues.

Passing the CompTIA A+ exam requires a basic understanding of key port numbers, including the following:

21: FTP (File Transfer Protocol) is the default control port that is used to transfer data between devices. When you initiate an FTP connection, your computer (the client) communicates with the FTP server's port 21 to establish the control channel. Once the control channel is established, the client can send commands to request files or upload files to the server. Web developers commonly use FTP to update various files for a website, including HTML, CSS, JavaScript, and image files.

22: SSH (Secure Shell) uses public key cryptography to open a secure network channel over an unsecured network. It is widely used in scenarios where secure, encrypted remote access to servers is required for tasks ranging from routine administration to troubleshooting and applying critical security updates. Many companies have a set of remote servers that host web applications, databases, and other services. SSH provides system administrators a secure way to access and manage these servers remotely.

23: Telnet is a network protocol for interactive text-based communication between two devices over a computer network. It’s important to note that Telnet does not encrypt data and due to security concerns related to transmitting data in clear text, it has been largely replaced by more secure alternatives like SSH in many modern network environments.

25: SMTP (Simple Mail Transport Protocol) uses TCP port 25 for Internet mail transmission. SMTP is an internet standard protocol. Due to the proliferation of SPAM and other email-related vulnerabilities, other ports can be used for SMTP, most notably port 587. A secure, nonstandard implementation of this protocol is SMTPS. SMTPS is quite effective because it is one of the protocols that can utilize SSL. SMTP is responsible for the transmission of email between servers and for sending email from a client. Other protocols are used by the client to receive email.

53: DNS (Domain Name System) is known for translating human-readable domain names (like www.example.com) into IP addresses that machines on the network can understand. When you enter a website's domain name in a web browser, your computer needs to find out the corresponding IP address to establish a connection. Your computer sends a DNS query to a server, asking for the IP address associated with the given domain name. In response, the DNS server looks up the information and sends back a response to your computer, including the IP address associated with the requested domain, allowing your computer to establish a connection with the web server hosting the website.

80: HTTP (Hypertext Transfer Protocol) is the foundation of data communication on the World Wide Web, and it enables the transfer of hypertext between web browsers and web servers. Port 80 is the default port for unencrypted HTTP traffic and plays a central role in facilitating the retrieval and rendering of web pages on the Internet.

110: POP3 (Post Office Protocol version 3) is a protocol for downloading emails from a mail server to a client device, such as a computer or smartphone. When an email client, like Outlook or Thunderbird, wants to retrieve emails from a mail server using POP3, it connects to the server on Port 110 and authenticates itself using a username and password. This allows the server to identify the specific mailbox associated with the user.

143: IMAP (Internet Message Access Protocol) is also used for retrieving emails from a mail server to a local email client. Unlike POP3, which typically downloads emails to the client and removes them from the server, IMAP allows users to manage their emails directly on the server. This includes organizing emails into folders, marking them as read or unread, and managing drafts and sent items. Port 143 is valuable because it supports synchronization between the email client and the mail server, ensuring changes made on one device are reflected across all devices and providing a consistent view of the mailbox across multiple platforms.

443: HTTPS (Hypertext Transfer Protocol Secure) is a secure version of HTTP, and is used to secure the communication between web browsers and web servers by encrypting the data transferred between them. HTTPS uses protocols like SSL (Secure Sockets Layer) or its successor, TLS (Transport Layer Security), to encrypt the data and prevent unauthorized parties from intercepting and reading the information exchanged between the user's browser and the web server.

Port Numbers in CompTIA A+ Certification

To pass the CompTIA A+ exam and earn the certification, you will have to learn port numbers and their functions. It’s a lot of memorization, but you can’t overlook the importance of understanding port numbers if you want to work in IT. A+ certified professionals should be able to troubleshoot basic network issues, and knowledge of port numbers is essential when diagnosing connectivity problems or configuring firewalls to allow specific types of traffic.

Memorization is key to passing this exam, and while learning port numbers can be challenging, it's an essential aspect of IT. Fortunately, for the purposes of the A+ exam, students won’t need to know all 65,535 ports. Instead, you will need to know the three blocks of ports and how they are divided: well-known (0 to 1023), registered (1024 to 49,151), and dynamic or ephemeral ports (above 49,151). From there, the exam will primarily focus on the well-known ports, as these are used every day (e.g., port 80 for web traffic and port 443 for encrypted web traffic).

Questions are often structured as follows:

What port number is used by SMTP (Simple Mail Transfer Protocol) when you're sending email?

Answer: Port 25

What company uses port 3074 for its online gaming service?

Answer: Xbox (for Xbox Live)

Practical Applications and Troubleshooting

Viewing and managing port numbers can be done through various tools and commands in different operating systems. Here's a basic guide for some commonly used operating systems:

Windows:

• Open the Command Prompt and use the command ‘netstat -an’ to display a list of all active network connections and listening ports.

• Open the Resource Monitor (this is located in the Task Manager under the "Performance" tab). Navigate to the "Network" tab to see the list of active network connections and associated ports.

*To manage ports related to Windows Firewall, open "Windows Defender Firewall with Advanced Security." Navigate to "Inbound Rules" or "Outbound Rules" to allow or block specific ports.*

 

Linux:

• Open the terminal and use the command ‘netstat -tuln’ to display a list of all listening ports.

• Use the ‘lsof -i’ command to list all open internet files (which includes open ports).

 

*Use tools like ‘iptables’ or ‘ufw’ to manage the firewall settings, including opening or closing specific port*

 

macOS

• Open the terminal and use the command ‘netstat -an | grep LISTEN’ to display a list of all listening ports.

• Use the ‘sudo lsof -iTCP -sTCP:LISTEN -n -P’ command to list all open TCP listening ports.

 

*Use the ‘pfctl’ command to manage the built-in packet filter firewall.*

 

The CompTIA A+ exam will require a rudimentary understanding of how to troubleshoot port-related issues in networks. To do that, you’ll have to know how to identify problems with the communication between devices and understand how data is transmitted through specific ports. Here are common steps to troubleshoot port-related issues:

 

• Ensure that there is a network connection between the devices.

• Verify device IP addresses and ensure they are on the same subnet and can reach each other.

• Check firewall settings on both the sending and receiving devices as firewalls may block traffic on specific ports. Adjust configurations to allow necessary communication.

• Use the ping command to test basic connectivity between devices. If there's no response, it might indicate a network connectivity issue.

 

Conclusion

Port numbers are vital for effective communication in networking, and understanding them is crucial for IT professionals seeking the CompTIA A+ certification. Every entry-level IT practitioner is expected to have these skills, so if you’re ready to get A+ certified, it’s time to delve deeper into port numbers. Learning what port numbers do and why they matter will help you provide troubleshooting, security, and application support.