Nmap Development: a small doubt

[Turbo <sandeep_kr () students iiit net>]

PS : I am new to this list. In case, my question is inappropriate here 
please ignore it or direct me to appropriate lists.

I am trying to find out a list of all FTP servers on my college LAN using 
this command:-
$ nmap -n -P0 -p 21 -oG ftpLog.gnmap -iL IPs.txt

It returns 110 results. But when I remove -P0 option, it returns only 84 
results. Why? Am I doing something wrong?

IPs.txt containes a range that specifies 8960 IPs.
The original command take around 10 mins to complete the scan, whereas the 
command without -P0 takes only 1.5 mins! Thats why I want to use this.

Sandeep Kumar
-- 
I do know everything, just not all at once. It's a virtual memory problem.
Home Page : http://students.iiit.ac.in/~sandeep_kr
Blog : http://sandy007smarty.blogspot.com

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