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Thread: Addressing

  1. Junior Member
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    #1

    Default Addressing

    Lets suppose that network 192.168.0.0/24 is external and we bought this 254 addresses.

    If we have an external class C net 192.168.0.0 255.255.255.0 and we try to split it to subnets containing more that 62 hosts we could have a problem.

    If we try to make a subnet containing more that 62 hosts it would be illegal. I mean changing only one bit.
    Probably.
    First illegal subnet
    198.168.1.0/255.255.255.128
    and second
    198.168.1.128/255.255.255.128
    Why illegal?
    The address of the first subnet i the same as the address of net.
    The broadcast address of the second subnet is the same as the broadcast address of our net.

    If we try to make legal subneting we would have to change not one but two bits. So we can have to subnets like this.
    1. 198.168.1.64/255.255.255.192
    2. 198.168.1.128/255.255.255.192

    But if the entire network is external and we are paying for every address this is a waste of money. We only can have two 62 host subnets.

    If for example we are a provider and out client wants bigger that 62 host subnet we would have to use illegal addressing or buy bigger class B network.
    I wonder is it possible to deal somehow with this problem and use illegal addressing.
    And if not then why? What kind of fatal errors can occur?
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  3. Johan Hiemstra Forum Admin Webmaster's Avatar
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    #2
    Here's an excerpt from a page at cisco that describes it very well: the use of the ip subnet-zero command to allow Subnet Zero and the All-Ones Subnet

    ""Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing. According to RFC 950 , "It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets." This is the reason why network engineers required to calculate the number of subnets obtained by borrowing three bits would calculate 23-2 (6) and not 23 (8 ). The "-2" takes into account that subnet zero and the all-ones subnet are not used traditionally.""

    CIDR solves it and would enable you to use the all zero network and all 1s broadcast address (for a separate subnet that is...). Cisco will mention either CIDR or ip subnet-zero in an exam question to let you know if you should subtract the '2' or not.

    In modern homogenous environments (ie. all cisco) you don't have to worry about this.

    Are you using outdated material to prepare for the exam?
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  4. Junior Member
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    #3
    First of all thanks for the link.
    My problem in Polish forum last for a year unsolved and hear one day is enough.
    Hmm this problem wasn't so big for me because I use only private addressing to make subneting.
    But since I have read about addressing this problem occurred in my mind and I simply wanted to know.
    As they say what if...

    Yes this was few years ago so you can say it was an old book now. Later I thought that I know everything about subneting (except this certain problem practically why not to use zero and all one subnets) so I don't have to read about it again just because a new book appeared.

    Now I know and it feels good.
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  5. Johan Hiemstra Forum Admin Webmaster's Avatar
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    #4
    You're most welcome

    The new 'subnetting' is defined in another RFC than the traditional method. For more information, here are two 'typical' subnetting discussions:

    www.techexams.net/forums/viewtopic.php?t=447

    www.techexams.net/forums/viewtopic.php?t=456

    If you are using an older book for CCDA, be sure to do some background reading about IPv6 and SAFE.
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