What is an open port?
A port is a numbered doorway on an IP address. Services listen on specific ports: websites on 80 and 443, secure shell on 22, remote desktop on 3389. A port is open if something is listening and the connection is allowed through, closed if the machine answers "nothing here", and filtered if there is no answer at all, which usually means a firewall is silently dropping the traffic.
Why check your own ports?
Most home routers block every incoming connection by default, which is the safe state. Ports become reachable when you (or software you installed) ask the router to forward them: for a game server, a camera, a NAS or remote access. Old routers and some devices also open ports automatically using UPnP. An unnoticed open port can expose a login page, a file share or a remote-control service to the entire internet, where automated scanners find it within minutes.
How this scanner works and what it will not do
- It scans only the public IP address your request came from. You cannot enter someone else's address, and the server ignores any address you send.
- It tests a fixed list of 24 common ports with a normal TCP connection attempt. It does not send exploits, guess passwords or probe services.
- It is limited to a few scans per half hour per network and refuses cross-site requests, so another website cannot trigger scans on your behalf.
- The result is shown to you and is not stored.
If you are on a VPN, mobile network or shared Wi-Fi, the address scanned belongs to the VPN, carrier or network operator, and you will almost always see closed or filtered ports. That tells you little about your own device. Test from a home connection with the VPN off if you want to check your router.
Ports that deserve attention
| Port | Service | Why it matters |
|---|---|---|
| 23 | Telnet | Unencrypted remote login. It should never be exposed. |
| 445, 139, 135 | Windows file sharing and RPC | Frequent target of worms and ransomware. |
| 3389 | Remote Desktop | Heavily attacked. Use a VPN rather than exposing it. |
| 3306, 5432, 1433 | Databases | Databases should not face the internet. |
| 5900 | VNC | Often weakly protected remote control. |
| 22 | SSH | Fine when hardened (keys, no password login), but heavily scanned. |
What to do if a risky port is open
- Log in to your router and look for port forwarding or virtual server rules. Remove any you do not recognise.
- Turn off UPnP if you do not need it.
- Update the router firmware and change default passwords.
- If you need remote access, put it behind a VPN instead of exposing the port directly.
- Scan again to confirm.
Last reviewed 2026-10-26. Found a mistake? Report a correction (how we handle them).