How to Check Open Ports: Windows, Linux and Online Tools

A port check tests whether a specific port on a device or server is reachable (open) from outside. The quickest methods are the Test-NetConnection command on Windows, nc on Linux and macOS, or an online port checker run against your own IP address.

In this guide we explain how to check open ports, what “open”, “closed” and “filtered” results mean and what to do if a port won’t open.

Note: only scan ports on your own devices or systems you are authorised to manage. Scanning other people’s systems without permission may breach terms of use and the law.

What Is a Port and Why Check It?

A port is a numbered gate used to separate the network traffic of applications on a device. For example, websites use ports 80 and 443, SSH uses 22 and Remote Desktop uses 3389.

A port check is usually needed:

• when a game or application server can’t be reached from outside;
• to confirm that port forwarding on your router works;
• to see, for security, whether unnecessary ports are left open on your server.

Port States: Open, Closed and Filtered

State	Meaning
Open	An application is listening on the port and accepting connections.
Closed	The device is reachable but no application is listening on that port.
Filtered	A firewall is blocking the packets; no response comes back.

Checking Ports on Windows

Testing whether a remote port is open

Open PowerShell and run:

Test-NetConnection server-address -Port 443

If the result shows TcpTestSucceeded : True, the port is open.

Seeing which ports are listening on your computer

netstat -ano | findstr LISTENING

To look for a specific port, use netstat -ano | findstr :3389. With the PID in the last column you can find which program is using the port in Task Manager.

Checking Ports on Linux and macOS

Remote port test

nc -zv server-address 22

A “succeeded” or “open” response means the port is open.

Listing listening ports on a server

sudo ss -tulpn

This command shows all ports listening on TCP and UDP and the services that opened them.

Scanning your own server with nmap

nmap -p 22,80,443 server-address

nmap checks several ports at once and reports whether each is open, closed or filtered.

Online Port Check Tools

If you’re checking the port of a device behind your router, the test has to be done from outside. Online port checkers try the IP and port you enter over the internet. For an accurate result:

• make sure the relevant application (game server, camera software, etc.) is running during the test;
• check that the IP shown by the tool matches your router’s external IP address.

What to Do If a Port Won’t Open

1. Is the application running? If no service is listening, the port shows as closed. Check first with netstat or ss.
2. Operating system firewall: add an inbound rule in Windows Defender Firewall. On Linux, define ufw allow 25565/tcp or a firewalld rule.
3. Port forwarding on the router: forward traffic from the external port to your device’s local IP address. Don’t forget to give the device a static local IP.
4. CGNAT check: if your router’s external IP differs from the IP you appear with on the internet, your ISP may be using CGNAT. Port forwarding won’t work in that case; you’ll need to request a static/public IP from your ISP.
5. Provider firewall: if your server is in a data center, there may be a separate network firewall in the panel. Open the rule there too.
6. The right protocol: some applications use UDP. A TCP test tells you nothing about a UDP port.

Port Checks for Security

• Keep only the ports you actually use open on your server.
• Protect management ports such as SSH and RDP with IP restrictions, and change the default port if needed.
• Don’t expose database ports (MySQL 3306, PostgreSQL 5432) to the internet; allow access only from local or private networks.
• Use a firewall on your Linux servers and review its rules regularly.

Frequently Asked Questions

Is checking ports legal?

Checking your own devices and the servers you manage is a common, legitimate administration task. Don’t scan other people’s systems without permission.

The port shows as open but I can’t connect. Why?

The application may only allow local connections in its own settings (e.g. listening on 127.0.0.1). Check that it listens on all interfaces (0.0.0.0).

Can I check a port with Telnet?

Yes; if telnet address port connects, the port is open. However, the Telnet client is disabled by default on Windows. Test-NetConnection is more practical.

How do I check UDP ports?

Because UDP is connectionless, a definite result is harder to get. You can scan with nmap -sU -p port address, but the result is often “open|filtered”. The most reliable test is to connect with the application itself.