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Subnetting in CCNA Exam

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  • 13-05-2010 12:24pm
    #1
    CabbageThing
    Registered Users Posts: 43 ✭


    Was just wondering if anyone has passed the CCNA exam if they can confirm whether or not Cisco sunbet questions allow the 1st and last subnets as valid subnets or do you have subtract 2 from the total number of subnets to get the actual number of usable subnets.
    E.G.
    IP address:172.20.100.1
    Class B (So 3rd and 4th Octets available for subnetting)
    subnet mask: 255.255.240.0

    So the the subnet mask in Binary 11111111.11111111.11110000.0000000
    Total number of subnets (3rd Octet)= 2power 4= 16
    or
    Total number of subnets 2power4= 16 - 2 = 14

    In some non Cisco practice questions the answer is 14 not 16
    In others the answer is 16

    What do cisco accept ?

    Here are some links for practicing subnetting - you will see what I mean if you start working through these
    http://www.subnettingquestions.com/

    http://www.scribd.com/doc/2158504/IP-Addressing-Subnetting-Workbook

    http://www.proprofs.com/forums/index.php?showtopic=4744


Comments

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    #2
    NullZer0
    Registered Users Posts: 1,629 ✭✭✭NullZer0


    Looks like the 2 to the power of 4 - 2 give you the 14.

    Which is for valid host addresses on the network i.e. 1 for Subnet, 1 for broadcast.


    HTH,


  • Options
    #3
    CabbageThing
    Registered Users Posts: 43 ✭CabbageThing


    Thanks iRock , I was on the cisco learning center playing the subnet game an elaborate but cool subnetting learning exercise and it accepted the first subnet - the subnet with all zeros in the network portion and it also accepted the last subnet - the subnet with all ones in the network portion.


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    #4
    Razzen
    Registered Users Posts: 152 ✭✭Razzen


    Older routers struggled when subnets where introduced because some of the protocols they used where still classful (they didn't include subnet information). so to avoid futher confussion the first and last subnets were not used.

    As of cisco ios version 10 (I think) this is no longer the case and Cisco now encourages the use of all subnets. So for your CCNA exams yes Subnet zero etc is always counted.

    Hope that helps
    :D


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    #5
    CabbageThing
    Registered Users Posts: 43 ✭CabbageThing


    Thanks Razzen , thats perfect


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    #6
    FruitLover
    Registered Users Posts: 2,534 ✭✭✭FruitLover


    Because of the confusion (old exams not accepting subnet zero), I don't think they ask questions anymore that would require you to explicitly say the number of subnets (i.e. decide whether to count subnet zero or not).

    Modern IOS versions have subnet zero enabled by default, and it's very commonly used in the real world.


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