A port is a numbered door on an internet connection. Web servers listen on port 443, email servers on port 25, and game servers and home apps on ports of their own. When you set up something that needs to be reached from outside, such as a game server, a security camera or a home media server, you need to know whether its port is actually open to the internet. The checker above tries to connect to a port on your own connection from our server and tells you whether it is open, closed or blocked, along with what to do next.

What an Open Port Is

Every device on the internet has an address, and each address has 65,535 ports. A program that wants to accept connections, like a web server or a game server, listens on a specific port. When another computer connects to your address on that port, the connection goes to that program. If nothing is listening, or a firewall blocks the port, the connection fails.

On most home connections, your router stands between your devices and the internet and blocks incoming connections by default. To let outside connections reach a device, you set up port forwarding on the router, which sends a port to one device on your home network.

What This Tool Checks

Enter a port number and the tool tries to open a connection to that port on the address you are visiting from. You can also check 20 common ports at once, including those used for websites, email, remote access, databases, Minecraft and Plex. For each port, the result is one of three states:

• Open: something accepted the connection, so the port is reachable from the internet
• Closed: your connection answered and refused, so nothing is listening on that port
• Filtered: nothing answered within three seconds, which usually means a firewall dropped the connection

For safety, the tool only ever checks the address you are visiting from. It cannot be pointed at anyone else’s connection.

How to Read Your Results

When you check a single port, a green verdict means the port is open and port forwarding is working. An amber verdict means the port is closed or filtered, and a How to Open It box walks through the usual fixes. When you check common ports, the verdict counts the open ones. Ports that are often attacked, such as Remote Desktop, VNC, databases and FTP, are marked in red if open, with advice on closing them.

Your public address is shown at the top of the tool. That is the address the rest of the internet sees, and it is usually your router’s address rather than your computer’s.

Why a Port Shows as Closed

If you set up port forwarding and the port still shows as closed or filtered, work through these causes in order. First, the program must be running and listening on that port, so start the game server or app before checking. Second, the router’s forwarding rule must point to the right device, using that device’s current local address, which can change unless you reserve it in the router settings. Third, the device’s own firewall, such as Windows Defender Firewall, must allow the port.

If all of that is correct, the problem may be outside your home. Some internet providers block common ports such as 25 and 80 on home plans. Others share one public address among many customers, a setup called CGNAT, which makes port forwarding impossible. You can usually spot CGNAT if the address shown here differs from the WAN address on your router’s status page. Your provider can often give you a public address on request.

Ports That Should Usually Be Closed

An open port is an open door, and automated scanners try every address on the internet many times a day. Some services are frequent targets. Remote Desktop on port 3389 and VNC on port 5900 give full control of a computer and are among the most attacked services online. Databases such as MySQL on port 3306 and PostgreSQL on port 5432 should almost never be reachable from the internet. FTP on port 21 sends passwords without encryption.

If any of these show as open and you did not set them up on purpose, remove the forwarding rule on your router and turn off the service. If you need remote access, a VPN is far safer than opening these ports directly.

Checking a Home Server or Game Server

For a Minecraft server, check port 25565. For Plex, check port 32400. For a home web server, check ports 80 and 443. If the port shows as open but friends still cannot connect, make sure they use your public address, shown at the top of the tool, and that your address has not changed, since many home connections get a new public address from time to time. A dynamic DNS service can give your connection a name that follows the address as it changes.

If you are behind a VPN while checking, the tool sees the VPN’s address instead of your home connection, so turn the VPN off for an accurate result.

Port Forwarding Step by Step

Router menus differ, but the steps are much the same everywhere. Sign in to your router, usually by typing its address, often 192.168.0.1 or 192.168.1.1, into a browser. Find the section called Port Forwarding, Virtual Servers or NAT. Add a rule with the port number, the protocol, which is TCP for most apps, and the local address of the device running the program. Save the rule, make sure the program is running, then check the port again here.

If your internet provider supplied a combined modem and router, and you also have your own router behind it, the port may need forwarding on both, or the provider’s device may need to be put into bridge mode.

IPv4 and IPv6

Many connections now have both an IPv4 and an IPv6 address. The tool checks whichever one your browser used to reach Domibot, shown at the top. Port forwarding on most routers only applies to IPv4, while IPv6 devices are reached directly through the router’s firewall, so a port can be open on one and closed on the other.

Your Checks Stay Private

We do not save your address, the ports you check or the results. Each check opens a connection, records whether it succeeded and closes it straight away without sending any data.