# 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:

- 192.168.0.0 / 24 subnet mask 255.255.255.0
- 192.168.1.0 / 24 subnet mask 255.255.255.0
- 192.168.2.0 / 24 subnet mask 255.255.255.0
- 192.168.3.0 / 24 subnet mask 255.255.255.0

Let’s convert these network addresses to binary:

192.168.0.0 | 11000000 | 10101000 | 00000000 | 00000000 |

192.168.1.0 | 11000000 | 10101000 | 00000001 | 00000000 |

192.168.2.0 | 11000000 | 10101000 | 00000010 | 00000000 |

192.168.3.0 | 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:

00000000 |

00000001 |

00000010 |

00000011 |

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 192.168.0.0 /22 (subnet mask 255.255.252.0).

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 255.255.252.0

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:

- 172.16.0.0 / 16 subnet mask 255.255.0.0
- 172.17.0.0 / 16 subnet mask 255.255.0.0
- 172.18.0.0 / 16 subnet mask 255.255.0.0
- 172.19.0.0 / 16 subnet mask 255.255.0.0
- 172.20.0.0 / 16 subnet mask 255.255.0.0
- 172.21.0.0 / 16 subnet mask 255.255.0.0
- 172.22.0.0 / 16 subnet mask 255.255.0.0
- 172.23.0.0 / 16 subnet mask 255.255.0.0

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 172.16.0.0 /13 (subnet mask will be 255.248.0.0).

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 255.248.0.0

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

- 172.16.0.0 /16 is one network.
- 172.16.0.0 /15 are two networks.
- 172.16.0.0 /14 are four networks.
- 172.16.0.0 /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.

Rene,

Thank you so much for your attention!!! Now I can understand better about this subject!

and about:

“The only thing to be aware of is that your summaries include networks that you “don’t have”.”

Yes, There is situation where is not possible to have a summarization so specific like example above, where we had that summarize 5 networks and it was need to use a block size of 8.

I’m really grateful/thankful with your explanations. Thanks, thanks and thanks!!!

Hug

Hi James,

Where did you find this example? It’s not on this page?

Rene

Rene,

I got it off a ccna test page, but I believe I figured out the solution, what threw me off what the 172.1.128.0/25, it’s similar to how you solved the other two.

thanks

James

Hi Lokesh,

There are a couple of potential issues.

192.168.0.0 /24

192.168.1.0 /24

192.168.2.0 /24

192.168.3.0 /24

If you would create a summary like 192.168.0.0 /20 then basically you are advertising the 192.168.0.0 - 192.168.15.0 range to other routers. When your router receives a packet with destination 192.168.6.6 or something it will drop it.

Asymmetric routing, This one is harder to explain with text only. When you advertise a summary it’s possible that other routers will select a different (more specific) path as the next hop.

Traffic engineering: If you use a routing protocol then you can play with the different metrics for each prefix. If you advertise a summary instead of all the different prefixes then you won’t be able to change metric (or BGP attributes) for a single prefix.

Rene

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:

192.168.1.0/28

192.168.1.16/28

These could be

summarizedas 192.168.1.0/27 because /27 is smaller than the natural /24 boundary.Now, if you had this:

192.168.0.0/24

192.168.1.0/24

The

supernet(or CIDR) of these would be 192.168.0.0/23 because /23 is greater than the natural /24 boundary.