6 Replies Latest reply: Jun 30, 2011 4:13 PM by Martin RSS

    learnkey subnetting need help






      I was wondering if anyone has seen or is comfortable in explaining, step by step how to use the following sub-net calculator? In the learnkey cd 5 they show how to sub-net with a class b address in detail which is fine, but when he does the reverse engineering he flies  through the 5 steps and builds on the previous steps to the point where it makes no sense to me at all. I totally understand the binary conversions but it is slow. and besides he said this would solve every sub-netting issue I will ever run into without doing math very quickly.  



      here is the calculator;


           128  192  224  240  248  252  254  255


           128   64    32   16     8     4     2      1


           255  127   63    31   15     7     3      1 




      I am taking the questions right out of chapter 12 pre-test.


      question 5 and/or 6,7,9 and/or 10.


      a how to of these examples would be great.

        • 1. Re: learnkey subnetting need help

          Hi Michael


          You can use all the magic solutions you want but it is far easier to learn your 16 times table and your powers of 2.


          Regards Conwyn

          • 2. Re: learnkey subnetting need help

            learnkey cd 5 !! have they released newer version recently?


            as far as I know recent learnkey CCNA training is from 2005 ! Very old and for expired exam.

            • 3. Re: learnkey subnetting need help
              block or Magic#1286432168420
              # of hosts126623014621na
              #of subnets248163264128256
              slash mask2526272829303132



              Magic number is multiplier; to get it from mask, subtract mask from 256, like 256 - 224 =32 32 is your Magic number;

              to get it from slash notation; substract slash number from 32, 32-29=3 then raise 2 to the power of that number;

              2^3=8 so 8 is your Magic number.


              Starting at 0, keep adding 32, 64, 96, 128, 160, 192, 224, 256

              in case of 8, 8, 16, 24, 32, 40, 48, .....256


              To find subent of IP address, just find between what 2 numbers (multipiers of magic number) lies your IP, the lower number is your subnet id


              To figure out the subnet number, divide IP by Magic # then multiply the whole number and magic number to get subnet id;

              f.e IP is,  take 223/32 =6.9...; take only 6 and multiply by 32 = 192
              IP of 223 is on 192 subnet;

              • 4. Re: learnkey subnetting need help

                Yes it is from 2005 but it should still be good for sub-netting, he doesn't explain it well enough for me. I thought someone may have used it. or understood what he was saying. This is what I mean, how do i use that chart to solve this problem?;



                q5) Which of the following IP addresses would not be in the same subnet as mask


                a -


                b -


                c -


                d -


                e -


                f -


                I know the answer is (F) this way;









                I'm trying to apply the chart to all the sub-netting questions in the cisco book chapter12. first I learn them the binary way or the drawn out way, so I know and understand what is going on, then using the chart for speed to get me through the test. but I am looking at using the particular chart from the learnkey cd's.


                thank you for the reply

                • 5. Re: learnkey subnetting need help

                  why do i need to memorize 16 times table? I saw your example under the VLSM and I am having trouble connecting the dots. powers of 2 have to be learned to find hosts and network but 16 times table?

                  • 6. Re: learnkey subnetting need help



                    Magic number  way:

                    1. magic # is 16;  256-240 = 16 in 3rd octet;

                    2. find a range of multiples of magic number that number from "interesting octet"   falls into;

                    0, 16, +16 =32 +16 =48 +16 =64, 80, 96 <<< 81 is between 80 and 96


                    so your range is 80 to 96;

                    take 1 less of 96 to get B-cast IP: 95.255; 1 less of that is last usable IP 95.254


                    IP of is on subnet /20

                    rang is .80.1 thru .95.254, B-cast .95.255


                    no need for Binary mess !