Scanning 65K ports on 45 subnets

I have Nessus running on 5 different machines, all running Suse 9.1.  I have
adjusted the following scan options:
Port range to all 65K+
Number of hosts to test at the same time = 10
Number of checks to perform at the same time = 5
Optimize the test = enabled
 
 
Prefs:
Simultaneous connections = 10
Network connection timeout = 2
Network Read/write timeout = 2
Wrapped service read timeout = 2
 
These 5 boxes range from P2 400mhz to P4 2.6ghz.  When I attempt to scan
even 1 subnet with each box, the scan time is ridiculous to the point of
making the results useless (typically 2 or 3 days, if it completes at all.)
Given that I have 45 class b subnets to scan, do any of you have any
suggestions for remedying this problem?  I have seen other posts on this
list where users are scanning with relatively low powered machines, and are
not having the speed issues that I am.  As an added note, scanning from my
OS X laptop I can scan one subnet in about 2 hours with the same settings.
 
 
Thanks, 
Chad McDonald, CISSP
Chief Information Security Officer
Georgia College & State University
478.445.4473  Office
478.454.8250 Cell
478.445.1202 Fax
 

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