how nmap can know my firewalled servers ?

["Alexey Eremenko" <al4321 () gmail com>]

Hi all !

I know that "nmap" can show open ports. But nmap also shows my
firewalled ports !
How?

Since some servers (like apache) are firewalled with iptables, how can
nmap know wherever
my system run the service with open port, filtered port or doesn't run
it at all ?