Ken’s Network Administration Information

How to Create the Chart

Here's how to create the chart below.

  1. First, write out the MAGIC line. This is a simple powers-of-two list starting at 128 and counting down. Everyone should be familiar with this from our initial days of converting binary to decimal and vice-versa.
  2. Write out the MASK line. Begin at the right with 255. Subtract the magic number below it to get the next number, 254. Write 254 in the next position to the left. Now repeat by subtracting the magic number, 2, from 254 to get 252. Write 252 in the next position to the left. Keep repeating the process until you have filled out the entire row to 128.
  3. Fill in the Classes with the slash numbers. We know a default Class A is /8 so the first number on the left will be /9. (Remember, this is for subnetting, so we start at the first subnet.) Simply count up from there, putting the next higher number in the next position to the right.
  4. Fill in Class B the same way. We know the default for Class B is /16 so the first number is /17. Fill in the rest.
  5. Fill in Class C the same way. We know the default for Class C is /24 so the first number is /25. Fill in the rest.
  6. Now we can go back and fill in the (hosts). Start at the bottom right with "1" and keep increasing by powers-of-two. I have them memorized up to 32768 myself. For the exams, that's as high as I need to remember.

Here's a video of me making the chart and explaining it as I go:

Here's a video of how to use the chart to determine subnetting information:

The Chart:

MASK 128 192 224 240 248 252 254 255
MAGIC 128 64 32 16 8 4 2 1
A /9 /10 /11 /12 /13 /14 /15/ 16
(hosts) 8388608 4194304 2097152 1098576 524288 262144 131072 65536
B /17 /18 /19 /20 /21 /22 /23 /24
(hosts) 32768 16384 8192 4096 2048 1024 512 256
C /25 /26 /27 /28 /29 /30 /31 /32
(hosts) 128 64 32 16 8 4 2 1