A Port Checker Tool is an online or software-based tool that checks whether a specific port is open, closed, or filtered on a device or server. Every internet service uses a port number. For example, websites use port 80 or 443, email services use ports like 25 or 587, and games use their own special ports. When a port is open, data can pass through it easily. When it is closed, the connection fails. A Port Checker Tool sends a small request to the selected port and waits for a response. Based on the response, it tells you whether the port is open or closed.