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  • Vlsm

    This isn't a Cisco specific question, but I have seen a recent subnetting thread here so thought it would be more valid in this forum than Misc.

    I am studying towards 70-291 (what a beast!) and have come accross a question in the MS book that I don't really understand. I am starting to read the excellent guide JeremyW post here, and am starting to view the tutorials and Learn to Subnet but was wondering if someone would mind explaining the question I am stuck on, so I have something to work on;

    You are going to subnet the network using VLSM. Your routers support the zero subnet and the longest match algorithm. Someone has suggested some network/subnet masks. Which are not valid?;

    The MS Press book states;

    Correct Answers: A, C, and E

    A- Correct: Mask =
    Increment = 256-128 = 128
    Because 128 is not a factor of 64 (64 is not divisible by 12, this network is not valid.

    B- Incorrect: Mask =
    Increment = 256-128 = 64
    64 is a factor of 192, so this network is valid.

    C- Correct: The Value in the fourth octet must be a power of 2, for example, 128, 64, 32 and so on. This network is not valid.

    D- Incorrect: Mask =
    Increment = 256-224 = 32
    32 is a factor of 96, so this network is valid.

    E- Correct: Mask =
    Increment = 256-240 = 16
    16 is not a factor of 8, so this network is not valid.

    F- Incorrect: Mask =
    Increment = 256 - 248 = 8
    8 is a factor of 40, so this network is valid.

    I am sure they are correct, but the explanations are not *enough* due to my lack of understanding.

    Thanks for reading.

  • #2
    Re: Vlsm

    A subnet mask shows which portion of the IP address is Network ID and which portion is Host ID. All the network ID must be to the left of the Host ID when written in binary.

    This is a valid subnet mask:

    1111 1111 * 1111 1111 * 1111 0000 * 0000 0000 (ones = network ID, zeroes = host ID)

    (FF.FF.F0.00) (

    These are not valid subnet masks:

    0000 1111 * 1111 1111 * 1111 0000 * 0000 0000 (Zeroes to the left of the Network ID)

    (0F.FF.F0.00) (

    1111 1111 * 1111 1111 * 1111 0001 * 0000 0000 (Ones in the Host ID portion)

    (FF.FF.F1.00) (

    1111 1011 * 1111 1111 * 1111 0000 * 0000 0000 (Zeroes in the Network ID Portion)

    (FB.FF.F0.00) (

    So you see the easiest way to check if a subnet mask is valid, is to convert it to binary and ensure that the network ID portion is a contiguous batch of ones to the left, and the host ID portion is a contiguous batch of zeroes to the right.

    If you don't know binary, then why on Earth not?!

    For my own and your protection, I do not provide support by private message under any circumstances. All such messages will be deleted and ignored.

    Anything you say will be misquoted and used against you


    • #3
      Re: Vlsm


      Thanks for your help- all is clear now!

      The organisation I am in at the moment isn't big enough to warrant subnetting, so I am a little rusty on the subject!

      Thanks again.