Forum Replies

  1. Rene,

    I have a doubt, all examples that you gave are “continuos” networks and an even number of networks.
    And when we have networks like below? I just can solve them with binary method. Is there another form?

    I choosed shortest and highest networks and convert them to binary, so the summary address will be a block size 64 networks. I can’t solve it using CIDR notation and block size method.

    Another example with an odd number of networks, I can solve it with binary method


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  2. Hi Gabriel,

    Good question, let’s look at these examples. First one:

    Let’s do it in binary first (in case someone else reads this):

    10 = 00001010
    20 = 00010100
    30 = 00011110
    40 = 00101000
    50 = 00110010

    Only the first 2 bits are the same. Our CIDR notation would be 8 + 8 + 2 = 18 bits and we’ll use network address

    This works but it’s slow…you can do it in decimal, just remember the block sizes:


    Now you only have to pick a block size that fits all of the netw

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  3. Hi Lokesh,

    There are a couple of potential issues.

    1. You can blackhole traffic. For example let’s say you have these 4 networks behind a router: /24 /24 /24 /24

    If you would create a summary like /20 then basically you are advertising the - range to other routers. When your router receives a packet with destination or something it will drop it.

    1. Asymmetric routing, This one is harder to explain with text only. When you advertise a summary it’s possible that other r

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  4. Oops! I made a mistake. The calculation of 6 (which means 6 bits are required for the summarization) is correct, but I applied those six bits to the “host” portion of the subnet mask and not the “network” portion. I have corrected my original post–thanks for pointing that out.

  5. Vikas,
    CIDR and Supernetting are the same thing. This is where you can aggregate networks together into larger networks beyond their natural network boundary.

    Although Supernetting/CIDR might be considered a type of summarization, “summarization” generally is considered to be constrained by natural (classful) network boundaries. The distinction is somewhat subtle. Suppose you have the following:

    These could be summarized as because /27 is smaller than the natural /24 boundary.

    Now, if you had this:

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