grue155

The TCP and UDP protocols define a response mechanism when a packet is delivered to a host with a closed port. Some operating systems provide a way to turn off that response (BSD systems and the "blackhole" sysctl setting, for example). Otherwise a firewall front-end running on the host that intercepts the packet and checks for a listening port. If no matching port exists, then the packet is dropped, and the operating system doesn't see it. With no error response, the machine is "stealthed", like it wasn't even plugged in.