Sorry I had a mistake in Figure A and B. It was fixed on 6/3/2006. Please download the fixed version and discard the first version.

I haven't found the need for any special subnet calculators since I memorize numbers like 2, 4, 8, 16, 32 and 128, 192, 224, 240, 248, 252, 254, 255. Using the "AND" operator in Windows Calculator in decimal mode is also VERY useful and I never understood why that's usually not taught. I feel sorry for people when I see them doing binary conversions on tests.

The ruler allows you to allocate subnets without a calculator. You just make sure the edge tick marks are always taller than the tick marks in the middle and you know it?s a clean subnet.