A port check tests whether a specific port on your public IP address is reachable from the internet. This is essential for anyone running a game server, web server, or remote access service — if the port isn't open, nobody can connect to you.
When you click "Check Port", Cloudflare's edge network attempts a TCP connection to your IP on the specified port. If the connection is accepted, the port is open. If it's refused, the port is closed. If there's no response, the port is filtered (firewalled).
The Security Scan checks ports that are commonly exploited by attackers — databases (MySQL, PostgreSQL, MongoDB, Redis), remote desktop (RDP, VNC), and legacy protocols (Telnet, SMB). If any of these are open to the internet without proper authentication, your system could be at risk.
Most home routers block all incoming connections by default (NAT firewall). If you've set up port forwarding for a game server or self-hosted service, a port checker confirms it's actually working. It also helps identify accidentally exposed services that could be security risks.