-PS command and Host Discovery

["Tony Cap" <tonycap26 () gmail com>]

Question concerning the -PS command and more. I may not be using it correctly but this is what I've been using to do 
weekly scans on our networks:

nmap -T4 -v -PS -sS -A -p- -oA d:\nmap\output\week3\vlanxx -iL d:\nmap\batch\week3\vlanxx.txt

Running nmap version 4.68 on a Windows Server 2003

I have referenced:
http://nmap.org/book/man-host-discovery.html

First off - am I using the -PS switch correctly?  Does this switch over ride the default or does it attempt to do host 
discovery both by deafult (TCP ACK) AND the TCP SYN flag set?


Second - Major differences on this particular vlan compared to others...  

One particular server has 30,000+ ports open with the -PS, only approx 30 with out it.
With the -PS I have only 4 host reporting on this vlan,  compared to 30 with out it.


Can I get a brief description of this switch and opinions on its use.  OR for that matter correct switches to be used 
when doing weekly host discovery.  Any input would be greatly appreciated.

ALSO - I'm using NDIFF to compare my weekly scans.  Great utility - looking forward to further updates on that...

Thanks!


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