Legend has it that once upon a time a networking instructor named Bob taught a class of students a method of subnetting any address using a special chart. This was known as the Bob Maneuver. These students, being the smart type that networking students usually are, added a row to the top of the chart and the Enhanced Bob Maneuver was born. The chart and instructions on how to use it follow. With practice, you should be able to subnet any address and come up with an IP plan in under a minute. After all, it's just math!
The Bob of the Enhanced Bob Maneuver was really a manager/instructor at SHL. He taught this maneuver to Bruce, who taught it to Chad Klymchuk. Chad and a coworker named Troy added the top line of the chart, enhancing it. Chad was first my instructor in Microsoft, then my coworker here at NAIT, and now is one of my Academy InstructorsI guess I am now his boss. And the circle is complete.
The Enhanced Bob Maneuver
Suppose that you have a Class C network and you need nine subnets.
On the bottom line (Number of Valid Subnets), move from right to left and find the closest number that is bigger than or equal to what you need:
Nine subnetsmove to 14.
From that number (14), move up to the line called Bit Place.
Above 14 is bit place 4.
The dark line is called the high-order line. If you cross the line, you have to reverse direction.
You were moving right to left; now you have to move from left to right.
Go to the line called Target Number. Counting from the left, move over the number of spaces that the bit place number tells you.
Starting on 128, moving 4 places takes you to 16.
This target number is what you need to count by, starting at 0, and going until you hit 255 or greater. Stop before you get to 256:
These numbers are your network numbers. Expand to finish your plan.
Range of Valid Hosts
1730 (17 is 1 more than network # 30 is 1 less than broadcast#)
31 (1 less than next network #)
Notice that there are 14 subnets created from .16 to .224.
Go back to the Enhanced Bob Maneuver chart and look above your target number to the top line. The number above your target number is your subnet mask.
Above 16 is 240. Because you started with a Class C network, the new subnet mask is 255.255.255.240.