Route Summarization

In this lesson we’ll take a look how you can configure summaries. First i’ll show you some examples how to do so in binary and then we’ll take a look at some tricks how you can do it in decimal (which is much faster).

Let’s say we want to create the most optimal summary for the following 4 networks:

  • / 24 subnet mask
  • / 24 subnet mask
  • / 24 subnet mask
  • / 24 subnet mask

Let’s convert these network addresses to binary: 11000000 10101000 00000000 00000000 11000000 10101000 00000001 00000000 11000000 10101000 00000010 00000000 11000000 10101000 00000011 00000000

Now we have to look how many bits these network addresses have in common. The first and second octets are the same, so that’s 16 bits.

Let’s zoom in on the third octet:


The first 6 bits of the third octet are the same. Now we have enough information to create our summary address.

8 + 8 + 6 = 22 bits

Our summary address will be /22 (subnet mask

Now you have seen how to do this in binary, let’s do it in decimal. There’s a simple trick you can use to calculate this summary.

As you can see we have 4 networks, or when we speak in ‘blocks’ it’s a block of 4. Here’s a formula you can use:

256 – number of networks = subnet mask for summary address.

For example: 256 – 4 networks = 252

The subnet mask will be

Another way to look at it is by using the CIDR notation. You know a /24 is a block of 256 addresses.  Using a /23 means you have 2 x 256, and a /22 means you have 4 x 256.

Let’s look at another example. Let’s say we want to summarize the following networks:

  • / 16        subnet mask
  • / 16        subnet mask
  • / 16        subnet mask
  • / 16        subnet mask
  • / 16        subnet mask
  • / 16        subnet mask
  • / 16        subnet mask
  • / 16        subnet mask

Let’s look at it in binary first. I’ll write down the second octet since the first one is the same for all network addresses:

16 00010000
17 00010001
18 00010010
19 00010011
20 00010100
21 00010101
22 00010110
23 00010111

The first 5 bits for all these addresses are the same. The first octet had 8 similar bits so that’s 8 + 5 = 13 bits.

The summary address will be /13 (subnet mask will be

Calculating in binary like this works but it’s slow. Let’s use our trick for this:

256 – number of networks = subnet mask for summary address.

So that’s 256 – 8 = 248. The subnet mask will be

We can also find it by just looking at the CIDR notations:

  • /16 is one network.
  • /15 are two networks.
  • /14 are four networks.
  • /13 are eight networks.

That’s a lot faster than looking at it in binary.

I hope this example is helpful for you to create summaries. Feel free to share this post! If you have any questions, feel free to leave a comment in our forum.


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


    ... Continue reading in our forum

  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. 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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  5. Hello Shivam

    Yes, you can summarize using classless masks. That’s part of what CIDR is (Classless Inter Domain Routing). For example, you can summarize the following networks

    using the following summarization:

    None of the subnet masks are classsful in this scenario.

    If you

    ... Continue reading in our forum

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