FULL PRODUCT VERSION :
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
There is a problem in the Java plug-in when using an automatic configuration script (.pac file) to determine the address of an HTTP proxy.
 
The following implementation of the FindProxyForURL function demonstrates the issue:
  
 function FindProxyForURL(url,host) {
	if (isInNet(10.132.64.151, "10.0.0.0", "255.0.0.0" )) {
		return "DIRECT";
	} else {
		return "PROXY proxy1:8080";
	}
}	

With 6u14:
When using the above code in the .pac file, the Java plugin will incorrectly use proxy1 as the HTTP proxy to download the html pages (as 10.132.64.151 does

belong to the 10.0.0.0 network considering the 255.0.0.0 mask).
  
With 6u13:
When using the above code in the .pac file, the Java plugin will correctly use "DIRECT".

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
see code in the description

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
see comment in the description
ACTUAL -
see description

ERROR MESSAGES/STACK TRACES THAT OCCUR :
see description

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
we have to use update 13 and are unable to upgrade to update 14 and above.

Release Regression From : 6u13
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.