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  • Can someone please explain this subnetting question

    Question: How many subnets and hosts can you get from the network 172.20.0.0/20?

    Answer: 16 subnets and 4094 hosts


    My Answer was 15 Subnets and 4094 hosts? - How is the answer 16??

    On the 3rd Octet the binary is 11110000 which means 4 bits are borrowed for the network ID and 4 plus the remainin octed 8 is used for the host ID

    4 = 15 subnets
    12 - 4096 (subtract 2 for the broadcase and network address)

    How did they get 16??

    Thanks

    Michael
    Michael Armstrong
    www.m80arm.co.uk
    MCITP: EA, MCTS, MCSE 2003, MCSA 2003: Messaging, CCA, VCP 3.5, 4, 5, VCAP5-DCD, VCAP5-DCA, ITIL, MCP, PGP Certified Technician

    ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

  • #2
    Re: Can someone please explain this subnetting question

    Yes but as you are borrowing the bits from the precedding octet then you can utilise all of the bits for use.

    Like this

    1. 0000
    2. 0001
    3. 0010
    4. 0011
    5. 0100
    6. 0101
    7. 0110
    8. 0111
    9. 1000
    10. 1001
    11. 1010
    12. 1011
    13. 1100
    14. 1101
    15. 1110
    16. 1111

    I always find it easier to write down, well within reason anyway, the available networks. I also used to use a table but for the life of me i can't remember what it was. I'll look out all my old notes and see if i can find it for you.

    Comment


    • #3
      Re: Can someone please explain this subnetting question

      I always thought you used the decimal notation i.e.

      1+2+4+8 for 4 bits = 15

      Cheers for clearing that up

      Michael
      Michael Armstrong
      www.m80arm.co.uk
      MCITP: EA, MCTS, MCSE 2003, MCSA 2003: Messaging, CCA, VCP 3.5, 4, 5, VCAP5-DCD, VCAP5-DCA, ITIL, MCP, PGP Certified Technician

      ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

      Comment


      • #4
        Re: Can someone please explain this subnetting question

        Using that way will work as well but you need to add 1 to it for the zero value, if that makes sense.

        In effect you are starting at 0001 and not 0000 and 0000 is your 16th subnet.

        I forgot how much i hated these questions

        Comment


        • #5
          Re: Can someone please explain this subnetting question

          Yeah that clears it up

          Thanks very much again

          Michael
          Michael Armstrong
          www.m80arm.co.uk
          MCITP: EA, MCTS, MCSE 2003, MCSA 2003: Messaging, CCA, VCP 3.5, 4, 5, VCAP5-DCD, VCAP5-DCA, ITIL, MCP, PGP Certified Technician

          ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

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          • #6
            Re: Can someone please explain this subnetting question

            NP m8. If you need any more help just holler.

            Comment


            • #7
              Re: Can someone please explain this subnetting question

              (2^n) - 2

              n being the number of bits.

              i.e. You have 12 host bits (2^12) - 2 = 4094

              You have 4 Network bits (2^4) - 2 = 14 (This is obviously if you are working out useable subnets, if they want the full figure just don't subtract 2 !!).

              topper
              * Shamelessly mentioning "Don't forget to add reputation!"

              Comment


              • #8
                Re: Can someone please explain this subnetting question

                > You have 4 Network bits (2^4) - 2 = 14

                the question is a bit strange to begin with. It has the unspoken assumption that you will want /24 networks (class C). BUT with that assumption you will have 2^(24-20) = 2^4=16 networks, not 14 of course. What is wrong with the 2 mystery networks you propose?
                Last edited by wkasdo; 10th January 2006, 12:59.

                Comment


                • #9
                  Re: Can someone please explain this subnetting question

                  Well the theory does state that the formula as (2^n)-2 which would give you the 14 subnets.

                  Now there was a reason why we include both those "un-usable" subnets but i can't remember what it was.

                  Useable ranges.

                  172.20.16.0 172.20.16.1 - 172.20.31.254 172.20.31.255
                  172.20.32.0 172.20.32.1 - 172.20.47.254 172.20.47.255
                  172.20.48.0 172.20.48.1 - 172.20.63.254 172.20.63.255
                  172.20.64.0 172.20.64.1 - 172.20.79.254 172.20.79.255
                  172.20.80.0 172.20.80.1 - 172.20.95.254 172.20.95.255
                  172.20.96.0 172.20.96.1 - 172.20.111.254 172.20.111.255
                  172.20.112.0 172.20.112.1 - 172.20.127.254 172.20.127.255
                  172.20.128.0 172.20.128.1 - 172.20.143.254 172.20.143.255
                  172.20.144.0 172.20.144.1 - 172.20.159.254 172.20.159.255
                  172.20.160.0 172.20.160.1 - 172.20.175.254 172.20.175.255
                  172.20.176.0 172.20.176.1 - 172.20.191.254 172.20.191.255
                  172.20.192.0 172.20.192.1 - 172.20.207.254 172.20.207.255
                  172.20.208.0 172.20.208.1 - 172.20.223.254 172.20.223.255
                  172.20.224.0 172.20.224.1 - 172.20.239.254 172.20.239.255

                  Actaully i think it was something to do with the bits being all zero's and all one's and that you can configure routers to use them. Could be wrong though.

                  Comment


                  • #10
                    Re: Can someone please explain this subnetting question

                    can someone please show me the working out for this one

                    Question: How many subnets and hosts can you get from the network 172.28.0.0 255.255.255.224?

                    Answer: 2048 subnets and 30 hosts

                    Michael
                    Michael Armstrong
                    www.m80arm.co.uk
                    MCITP: EA, MCTS, MCSE 2003, MCSA 2003: Messaging, CCA, VCP 3.5, 4, 5, VCAP5-DCD, VCAP5-DCA, ITIL, MCP, PGP Certified Technician

                    ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                    Comment


                    • #11
                      Re: Can someone please explain this subnetting question

                      Right.

                      Using the formula given (2^n)-2 you would get.

                      255.255.255.224 which leaves 5 bits in the host portion of the address.

                      So (2^5)-2 or (2*2*2*2*2)-2 = 30 Hosts

                      Now the network portion is basically a class b address subnetted to a class c which borrows 3 bits from the 4th octet. So effectively from a class b address you have a full octet ( + the three borrowed bits (8+3=11).

                      Now the calculation will look like this

                      (2*2*2*2*2*2*2*2*2*2*2)-2 = 2046. Now if you remebr we don't subract the 2 from the network portion so the real answer is 2048.

                      HTH

                      Comment


                      • #12
                        Re: Can someone please explain this subnetting question

                        Ahhh - I think it's finally sunk in.

                        Hope I dont get too many question on this

                        Thanks again

                        Michael
                        Michael Armstrong
                        www.m80arm.co.uk
                        MCITP: EA, MCTS, MCSE 2003, MCSA 2003: Messaging, CCA, VCP 3.5, 4, 5, VCAP5-DCD, VCAP5-DCA, ITIL, MCP, PGP Certified Technician

                        ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                        Comment


                        • #13
                          Re: Can someone please explain this subnetting question

                          Originally posted by m80arm
                          Ahhh - I think it's finally sunk in.

                          Hope I dont get too many question on this

                          Thanks again

                          Michael

                          I think the only stumbling block you'll come across is the network portion and removing the 2 extra subnets.

                          Once you get the hang of it it is quite easy.

                          The tutor we had wanted us to work them out within 2 minutes.

                          Comment


                          • #14
                            Re: Can someone please explain this subnetting question

                            I think where I was going wrong was not identifying the default subnet class. I was only counting the remainin bits in one octet thus sometimes missing out a full octet for the network ID

                            I think I've got the hang of it. Chances are I will probably forget after reading through the book and just before the exam

                            Thanks again

                            Michael
                            Michael Armstrong
                            www.m80arm.co.uk
                            MCITP: EA, MCTS, MCSE 2003, MCSA 2003: Messaging, CCA, VCP 3.5, 4, 5, VCAP5-DCD, VCAP5-DCA, ITIL, MCP, PGP Certified Technician

                            ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                            Comment


                            • #15
                              Re: Can someone please explain this subnetting question

                              Originally posted by m80arm
                              I think I've got the hang of it. Chances are I will probably forget after reading through the book and just before the exam

                              No probs m8. Glad to help.

                              BTW you will forget it if you don't use it and its only when you need to that you will pick it up again.

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