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How does nmap/zenmap determine a port is filtered?

How does nmap/zenmap determine that specific ports are "filtered" as opposed to just not being presented via a daemon on a host?
ZenMap1.GIF
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amigan_99
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amigan_99
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Hugh FraserConsultantCommented:
Nmap returns a "closed" status for a probe that receives a RST in response to a connect request. If there's a firewall in the way (either on the host itself or as a network device), a different response is returned, or none at all if the packets are simply dropped, generally resulting in a "filtered" status.. The Nmap page at http://nmap.org/book/man-port-scanning-basics.html lists the various status values that can be returned.

Nmap has many different port scanning techniques to try to derive more information from the scan.They're documented in http://nmap.org/book/man-port-scanning-techniques.html.
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amigan_99Author Commented:
Excellent - thank-you!
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