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A port is the number that tells a device which program a connection is for: 443 for a web server, 22 for SSH, 25565 for a Minecraft server. When you run something at home that others should reach — a game server, a security camera, a NAS, a remote desktop — your router has to forward the port from its public address to the device inside your network.

This tool answers one question: is that port reachable from the internet right now? Our server opens a single TCP connection to the port you choose on the address your request comes from and reports what happens:

• Open — something completed the TCP handshake. The service is reachable.
• Closed — your connection answered with a refusal (RST): the router or the device is there, but nothing listens on that port, or the forward points to the wrong device.
• No answer (filtered) — nothing came back within three seconds. A firewall drops the packets, the router does not forward the port, or you are behind carrier-grade NAT (CGNAT): many customers share one public IPv4 address, so incoming connections cannot be forwarded to you at all. Mobile connections are almost always behind CGNAT, and some fixed-line providers use it too.

The tool never tests another address: a port checker that accepts any target is a scanner for other people’s networks. Only TCP can be tested; UDP services do not answer an unsolicited probe.