🚪 Ports & Common Services

📘 What You Will Learn

• What a port is and why it exists
• Port numbers and the three ranges
• The most common ports and the services behind them
• TCP ports vs UDP ports
• How to see open ports on Windows and Linux
• How to test if a port is reachable
• How to fix the most common port errors

🧠 What Is a Port?

An IP address finds the device. A port finds the program on that device.

A port is a number from 0 to 65535. Every program that uses the network listens on a port.

A service is a program that waits for requests. For example, a web server is a service. It listens on a port and answers when someone connects.

👉 Ports work at the Transport Layer.

Simple meaning:

Your laptop  --->  93.184.216.34 : 443
                   (IP address)   (port)
                   which device   which program

📦 Easy Analogy

Think of a big apartment building.

Building address  =  IP address
Apartment number  =  Port
Person inside     =  Service (program)
Delivery          =  Your data

The building address alone is not enough. The delivery person also needs the apartment number.

🤔 Why Do We Need Ports?

One device runs many network programs at the same time: a browser, email, a game, a remote login.

Without ports, the device would not know which program should get each packet.

The port number is the label that says: "This data is for the web server, not for email."

🔢 Port Number Ranges

When you open a website, your device picks a random dynamic port for itself. The website's side uses 443.

📋 Common Ports & Services

💡 These are standards, not laws. A service can be moved to another port, but most people keep the default.

⚖️ TCP Ports vs UDP Ports

Port 80 over TCP and port 80 over UDP are two different doors.

🛠️ See Your Open Ports

A port that is listening is open and waiting for connections.

Windows

Open Command Prompt and run:

netstat -ano

Expected output (sample, yours will differ):

  Proto  Local Address        Foreign Address      State          PID
  TCP    0.0.0.0:135          0.0.0.0:0            LISTENING      1048
  TCP    192.168.1.10:50123   142.250.77.14:443    ESTABLISHED    8820

• LISTENING = the port is open and waiting
• ESTABLISHED = an active connection
• PID = the ID of the program using it

Linux

Open Terminal and run:

ss -tuln

What the letters mean: t = TCP, u = UDP, l = listening only, n = show numbers.

Expected output (sample, yours will differ):

Netid  State   Recv-Q  Send-Q   Local Address:Port   Peer Address:Port
tcp    LISTEN  0       128      0.0.0.0:22           0.0.0.0:*
udp    UNCONN  0       0        127.0.0.53%lo:53     0.0.0.0:*

📡 Test If a Port Is Reachable

Windows (PowerShell)

Test-NetConnection google.com -Port 443

Expected output (sample):

ComputerName     : google.com
RemotePort       : 443
TcpTestSucceeded : True

True means the port is open and reachable.

Linux

nc -zv google.com 443

Expected output (sample):

Connection to google.com 443 port [tcp/https] succeeded!

If nc is missing, install it with sudo apt install netcat-openbsd.

❌ Common Errors & Solutions

💡 Pro Tip

Every open port is a possible way in for an attacker.

Close the ports you do not use. Fewer open ports means a smaller attack surface (the number of ways someone could attack you).

🎯 Final Summary

- IP address finds the device, port finds the program
- Ports are numbers from 0 to 65535
- 0-1023 well-known, 1024-49151 registered, 49152-65535 dynamic
- Common ports: 22 SSH, 53 DNS, 80 HTTP, 443 HTTPS, 3306 MySQL
- LISTENING = open and waiting for connections
- Windows: netstat -ano | Linux: ss -tuln
- Test a port: Test-NetConnection (Windows) | nc -zv (Linux)
- Best practice: close ports you do not use

📌 Next Tutorial

NAT and Port Forwarding: how traffic from the internet reaches one device inside your home network.

TechWithJuned · Learn → Execute → Build