subnet calculation

Discussion in 'Cisco' started by w00, Feb 3, 2007.

  1. w00

    w00 Guest

    Hey, i'm making a test for cisco (CCNA01), but im at a question where
    i have to calculate subnets.
    But im not sure how to do this.

    The question is as follows:
    A company with a Class B license needs to have a minimum of 1000
    subnets with each subnet capable of accommodating 50 hosts. Which mask
    below is the appropriate one?

    A - 255.255.0.0
    B - 255.255.240.0
    C - 255.255.255.0
    D - 255.255.255.192
    E - 255.255.255.224

    I know the answer is D (255.255.255.192), but i would really like to
    know how i could find out why D is the correct answer.
    Could someone please help me out with the calculations.

    Thanks for any help.
     
    w00, Feb 3, 2007
    #1
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  2. w00 wrote:


    because you need at least all the third octet (255) times 4 (the borrowed
    two bits of the fourth one) to accommodate for at least the 1000 subnets
    you need. 255.255.255.192 leaves you with 62 useable hosts which is more
    than you need.

    --

    Cheers!
    D~
    http://www.telecom-gear.com



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    telecom-gear.com, Feb 3, 2007
    #2
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  3. w00

    Scooby Guest

    FYI....

    While 2^n-2 still applies for number of hosts, that is incorrect to use that
    formula for the number of subnets. With ip subnet-zero enabled (the
    default), you can use the lower and upper subnets, which gives you 2^n. You
    may have pulled that information from some older Cisco documentation, as
    that used to be the correct answer. Go figure.
     
    Scooby, Feb 3, 2007
    #3
  4. w00

    Drake Guest

    You have a class B address or /16 address so you have
    16 bits available for subnetting. a simple formula to help
    you determine how many bits to use for subnets and hosts
    is:
    2^n -2

    This implies you need 10 bits for 1000 subnets (2^10 - 2 = 1022)
    and the remaining 6 bits will give you enough hosts (2^6 -2 =62)
    to cover the stated requirement.

    Your subnet mask will be /26 or (11111111 11111111 11111111 11000000)
    255 . 255
    .. 255 . (128+64)
     
    Drake, Feb 3, 2007
    #4
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