NMAP Accuracy vs. Speed

[jburzenski () americanhm com]

I'm looking for any recommended settings for scripting nmap to accurately
scan several large logically disperse networks.  My target list consists of
about 5 c-class networks that do not respond to ICMP and about 40 single Ips
that either do or do not respond to ICMP.  

I would prefer accuracy over speed if the scan times are reasonable.  I have
been -P0 on all scans since I can't reliably detect 100% of online hosts
(which generates about 1.5MB of output in -oG format due to all of the
filtered ports reported on).  Scans with -T 3 currently take about 8 days.
Scans on -T 4 take about 1 day.  T 3 are noticeably more accurate and some
networks report as no hosts online or ports open under T 4.  

Here is my current scanning command:

nmap -T 4 -iL targetlist.txt -sT -P0 -oG output.log

My goal is to maintain an updated list of target networks and use nmap to
detect all open ports across the span monthly.  Then, another script will
diff the outputs and report on any changes.  Eventually, I'll get a udp
audit going as well. 

Any help would be appreciated.  



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