Common TCP & UDP Ports

Nerd Caf

Lesson Objectives

By the end of this lesson, you will:

• Understand the importance of TCP & UDP ports in networking.

• Learn the common TCP and UDP port numbers.

• Differentiate between well-known, registered, and dynamic ports.

• Perform practical exercises using command-line tools to check open ports and active connections.

1. Understanding TCP and UDP Ports

What is a Port?

• A port is a virtual endpoint for communication in a network.

• Ports allow multiple services to run on a single device using different identifiers.

• Every port is associated with an IP address and a specific transport protocol (TCP or UDP).

What is the Difference Between TCP and UDP?

Feature	TCP (Transmission Control Protocol)	UDP (User Datagram Protocol)
Connection	Connection-oriented (requires handshake)	Connectionless (no handshake)
Reliability	Reliable (ensures data delivery)	Unreliable (no guarantee of delivery)
Speed	Slower (due to error checking)	Faster (less overhead)
Use Cases	Web browsing, email, file transfer	Streaming, VoIP, gaming

2. Well-Known TCP & UDP Ports

Ports 0 – 1023 (Well-Known Ports)

These are assigned by the Internet Assigned Numbers Authority (IANA) for common services.

Port Number	Protocol	Service	Description
20 & 21	TCP	FTP	File Transfer Protocol (Data & Control)
22	TCP	SSH	Secure Shell for remote login
23	TCP	Telnet	Unsecure remote access
25	TCP	SMTP	Simple Mail Transfer Protocol (Email sending)
53	UDP/TCP	DNS	Domain Name System (Name resolution)
67 & 68	UDP	DHCP	Dynamic Host Configuration Protocol
69	UDP	TFTP	Trivial File Transfer Protocol
80	TCP	HTTP	Hypertext Transfer Protocol (Web browsing)
110	TCP	POP3	Post Office Protocol (Email retrieval)
143	TCP	IMAP	Internet Message Access Protocol
161-162	UDP	SNMP	Simple Network Management Protocol
443	TCP	HTTPS	Secure Web Browsing
3389	TCP	RDP	Remote Desktop Protocol

Ports 1024 – 49151 (Registered Ports)

• Used by software applications (e.g., database servers, proprietary apps).

• Examples:

  • 1433: Microsoft SQL Server

  • 1521: Oracle Database

  • 3306: MySQL Database

  • 5060: SIP (VoIP)

Ports 49152 – 65535 (Dynamic/Ephemeral Ports)

• Assigned dynamically by the OS for client connections.

• Used temporarily during communication.

3. Practical Exercises

Let's use real-world commands to identify and manage ports.

Exercise 1: Checking Open Ports on Your System

Windows:

• Open Command Prompt (cmd) as Administrator.

• Run:

netstat -an | findstr LISTENING

This will show active listening ports.

Linux/macOS:

• Open Terminal.

• Run:

netstat -tulnp

This will list open ports and associated services.

Exercise 2: Checking a Specific Port

Windows (PowerShell):

To check if port 80 (HTTP) is open:

Test-NetConnection -ComputerName google.com -Port 80

Linux/macOS:

nc -zv google.com 80

If the port is open, you will get a successful connection message.

Exercise 3: Scanning for Open Ports

Using Nmap (Install Required)

• Install Nmap:

  • Windows: Download from nmap.org

  • Linux/macOS: Use:

sudo apt install nmap  # Debian-based
sudo yum install nmap  # RedHat-based

• Scan a network for open ports:

nmap -p 1-1000 192.168.1.1

This checks for open ports on a local network device.

4. Key Takeaways

• TCP is reliable but slower; UDP is faster but less reliable.

• Well-known ports (0-1023) are assigned for critical services.

• Use netstat and nmap for port analysis.

• Security tip: Close unnecessary ports to reduce vulnerabilities.

Keywords

TCP ports, UDP ports, well-known ports, registered ports, dynamic ports, netstat, nmap, network security, port scanning, SSH, FTP, HTTP, HTTPS, SMTP, DNS, DHCP, SNMP, Telnet, RDP, firewall, نتورک پلاس