You set up a Minecraft server, a camera, remote access or a website at home, and nobody outside can connect. The first question is always the same: is the port open? This guide shows how to find out in under a minute and what to do with each result.
What a port is
The IP address gets data to your device; the port tells it which program the data is for. Websites use port 443, Windows Remote Desktop uses 3389, Minecraft uses 25565. An "open" port means a program is listening on it and nothing along the way is blocking the connection.
The fastest way: test from outside
Testing from inside your own network is misleading, because the traffic never even goes through the router. The test has to come from the internet. To do that:
- Keep the program or service running (the game server, the camera, the remote access).
- Open the port checker. The field is already filled in with your public IP.
- Pick the port from the list and click Test port.
How to read the result
- Open: it works. Anyone on the internet can connect to that port.
- Closed: the connection reached the device but was refused. Usually the program isn't running or is listening on a different port.
- No response: nothing came back. This is usually a firewall dropping the connection, or a missing forwarding rule on the router.
The port won't open: the checklist
1. Is the program running?
It sounds obvious, but it's the most common cause. With nothing listening, the port will never show as open. Also make sure it really uses the port you're testing.
2. Is there a forwarding rule on the router?
The router receives the public IP and shares it among the devices in your home. For an outside connection to reach the right computer, you need a port forwarding rule (also called NAT or "virtual server", depending on the brand) pointing the port to the device's internal IP, something like 192.168.0.10. Give that device a fixed IP on the local network so the rule doesn't break when it restarts.
3. Is the computer's firewall allowing it?
Windows Firewall and many antivirus programs block incoming connections. Allow the program or create an inbound rule for the port.
4. Does your connection use CGNAT?
Many ISPs, especially fiber and mobile providers, share a single public IP among several customers. In that case, no setting on your router will open the port. Signs of CGNAT: the IP shown on What is my IP is different from the WAN IP shown in the router's admin panel, or the router's IP falls between 100.64 and 100.127. The fix is to ask your ISP for a public IP, sometimes sold as a static IP.
5. Is your ISP blocking the port?
Some ISPs block ports such as 25 (outgoing email) and 80 on residential plans. Try an alternative port, like 8080, to confirm.
Security: open only what you need
Every open port is a way in. Open only what you need, keep the program updated and use a strong password. Avoid leaving remote access (3389) and databases (3306, 5432, 27017) open to the whole internet: these services are constantly targeted by bots trying to break in. If you can, use a VPN to reach your network from outside.
Summary
Test from outside, read the result and follow the checklist: program running, router port forwarding, computer firewall, CGNAT and ISP blocking. In the vast majority of cases, the answer is one of those five things.