Port Scanner

A Port Scanner checks whether specific TCP ports on a target host are open, closed, or filtered by attempting to establish connections to them. Every network service listens on a numbered port, such as 80 for HTTP, 443 for HTTPS, 22 for SSH, 25 for SMTP, or 3306 for MySQL. By probing a curated list of common ports, this tool reveals which services a host is exposing to the internet, giving you a quick inventory of its externally reachable attack surface.

The scanner works by initiating a connection handshake to each port in the list. If the port accepts the connection it is reported as open, meaning a service is actively listening. If the connection is refused the port is closed, and if no response comes back within the timeout the port is reported as filtered, which usually indicates a firewall is silently dropping the packets. Scanning only well-known ports keeps the check fast and avoids the noise of a full 65,535-port sweep.

Common use cases include verifying that a newly deployed server only exposes the ports you intend, confirming a firewall rule is actually blocking a port, and troubleshooting why a service is unreachable. Administrators use it after configuration changes to ensure no management ports like SSH, RDP, or database ports were accidentally left open to the public internet. It is also useful for confirming that a service you expect to be running, such as a mail server on port 25, is actually accepting connections.

Only scan hosts and networks you own or have explicit permission to test, since unauthorized port scanning may violate acceptable-use policies. An open port is not automatically a vulnerability, but unexpected open ports warrant investigation. Pair this tool with the SSL Certificate Checker to inspect what is running on port 443, the Mail Server Test for a deeper look at port 25 and STARTTLS, and a HTTP Headers check to examine what a web service on an open port returns.