[CentOS] Strange TCP ports phenomena

Hello list.

I have new server on CentOS 4.1 - fresh installation. During security
tests I've noticed:

When I scan server ports (nmap) from the outside there is 21 tcp port
open. But when I check on the server (netstat -tan or lsof -i) there is
no any open 21 tcp port.

Any ideas? To be honest I'm confused.

Regards

P.S.: of course I don't have started FTP service. Even I don't have
installed FTP server.
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