How to Check if a Port is Open (and How to Open It)

When a service isn’t reachable, checking if the correct port is open and listening is one of the first things to do. In this guide, we’ll show you how to:

• Test if a port is open using telnet or other tools
• Understand the role of listening services
• Open a port on Windows or Linux

🧠 Before You Start: What Does “Open Port” Really Mean?

A port is considered "open" if:

• It's not blocked by a firewall
• A program is actively listening on that port

If no software is bound to the port, or if a firewall blocks it, it will appear closed, even if technically available.

🧪 How to Test if a Port is Open

📟 Method 1 — Using Telnet

Telnet is a simple way to test if a port responds.

On Windows or Linux:

telnet your.server.ip port

✅ If the port is open and a service is listening, the screen will go blank or show a banner.
❌ If not, you’ll see a connection error like “Connection refused” or “Could not open connection”.

🧪 Example:

telnet 192.168.1.100 22

🛠 Method 2 — Using Netcat (Linux)

If telnet isn’t available, you can use nc (Netcat):

nc -vz your.server.ip port

✅ You’ll see a success message if the port is open.
❌ Otherwise, it will time out or be refused.

🌐 Method 3 — Online Port Check Tools

You can also use an online port checker like:

• https://portchecker.co
• https://yougetsignal.com/tools/open-ports

⚠ These tools only work if the port is open to the public internet (not behind NAT or local firewall).

🔓 How to Open a Port

🪟 On Windows

1. Open the Start Menu, type Windows Defender Firewall, and open it.
2. Click Advanced Settings on the left.
3. Click Inbound Rules > New Rule...
4. Choose Port, then click Next.
5. Select TCP or UDP, and enter your port number.
6. Choose Allow the connection, then click Next to finish the rule.

✅ Your port is now open in the Windows Firewall.

🐧 On Linux

Using UFW (Ubuntu):

sudo ufw allow 8080/tcp
sudo ufw reload

Using Firewalld (CentOS, RHEL, Alma, Rocky...):

sudo firewall-cmd --add-port=8080/tcp --permanent
sudo firewall-cmd --reload

✅ The port is now open in your Linux firewall.

🔁 Don't Forget: A Program Must Be Listening

Even if a port is open, it won’t respond unless something is listening on it.

Check with:

# Linux
sudo netstat -tulnp | grep :8080

# Windows (CMD)
netstat -an | find ":8080"

✅ If you see a result like LISTENING, it means the port is in use.
❌ If not, your app might not be running or bound to the right port.

🎉 In Summary:

• A port must be open AND have an active service to be reachable
• Use tools like telnet, nc, or netstat to check
• Don't forget to configure the firewall on Windows or Linux
• Always make sure something is actually listening on the port!

Updated on: 03/04/2025