Answer
PROBLEM
When scanning a machine, more ports are reported as open than there actually are.ENVIRONMENT
- NetGear Cable/DSL router
SOLUTION
Refer to your router documentation in order to configure it not to intercept packets and respond to them itself.CAUSE
This situation was encountered when using a NetGear Cable/DSL router and NAT was being used. Instead of forwarding the packets to the scanned machine, the NetGear router intercepts the packets and will respond back.Because of this one or more TCP or UDP ports will be marked as open when the machine is scanned.
It is not excluded that this problem can exist in other routers or other similar hardware.