how to find out a list of available ftp servers on LAN

[Turbo <sandeep_kr () students iiit net>]

I am using following command :-
nmap -n -P0 -p 21 -oG ftpLog.gnmap -iL IPs.txt
Are their other better options? or improvements to above command?
It takes around 7 mins to scan a list of 8960 IPs. Its ok, but I am still 
looking for faster options.

--
I do know everything, just not all at once. It's a virtual memory problem.
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