What can you infer if all results of a port scan show filtered ports?

When a port scan yields filtered ports, the most likely inference is that a firewall is in place. Firewalls are designed to control incoming and outgoing network traffic, and one of their primary functions is to filter ports based on defined security rules. When a port is marked as filtered, it typically means that the scanning tool was unable to determine whether the port is open or closed due to the firewall actively blocking the scan's probing attempts.

This indicates that the firewall is effectively preventing access to the ports being scanned, thus obscuring their status. As a result, the firewall enhances the security posture of the target system by restricting unwanted access, making it harder for potential attackers or unauthorized users to gain information about the system's network services.

Other potential interpretations, such as ports being closed without filtering mechanisms, would yield different scan results, typically indicating that the ports are merely closed rather than filtered. Similarly, if the target were not operational, the scan might not return any responses at all. Furthermore, the idea that ports are intentionally hidden from scanning also implies a form of filtering, again pointing back to the presence of a firewall or similar security device. Thus, the presence of filtered results most reliably suggests the existence of a firewall protecting the system.