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What is a Filtered Port?

A filtered port is a network port that cannot be properly checked because something is blocking access to it. Usually, a firewall, router, or security system is blocking the probe, so the scanner can’t tell whether the port is open or closed.

When a tool like Nmap scans a port, it sends a packet and waits for a response:

  • Open port → responds (service is listening)
  • Closed port → responds with rejection
  • Filtered port → no response at all

This usually happens because a firewall drops the packet silently, a router or security device blocks the traffic, or network rules prevent replies from being sent back.

A filtered result doesn’t mean the port is closed. It simply means the port scanner can’t see what’s behind the barrier. The port could be open, closed, or hidden.

Why do filtered ports exist? Filtered ports are generally good for security. They hide services from attackers, prevent reconnaissance (like port scanning) and enforce network access rules.