# Route Summarization

This topic is to discuss the following lesson:

Rene,

I have a doubt, all examples that you gave are â€ścontinuosâ€ť 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?

172.16.10.0/24
172.16.20.0/24
172.16.30.0/24
172.16.40.0/24
172.16.50.0/24

I choosed shortest and highest networks and convert them to binary, so the summary address will be
176.16.0.0/18 a block size 64 networks. I canâ€™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

192.168.0.0 / 24
192.168.1.0 / 24
192.168.2.0 / 24
192.168.3.0 / 24
192.168.4.0 / 24

The summary address will be 192.168.0.0/21, but when I was writing this example I saw that if I use block
size itâ€™s able solve it too. Itâ€™s need always think in block size like â€śpowers of 2â€ť.

When you have a free time, please, detail below

8 + 8 + 6 = 24 bits

Hug and thanks for yours articles. Itâ€™s helping a lot

1 Like

Hi Gabriel,

Good question, letâ€™s look at these examples. First one:

172.16.10.0/24
172.16.20.0/24
172.16.30.0/24
172.16.40.0/24
172.16.50.0/24

Letâ€™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 weâ€™ll use network address 172.16.0.0.

This works but itâ€™s slowâ€¦you can do it in decimal, just remember the block sizes:

2,4,8,16,32,64,128.

Now you only have to pick a block size that fits all of the networks that you want. The only block size that fits your networks is 64 or 128. Weâ€™ll try to be as specific as possible so weâ€™ll go for the 64.

Now you only have to figure out the subnet mask, just use this trick:

256 - block size = subnet mask.

So thatâ€™ll be 256 - 64 = 192. The subnet mask will be 255.255.192.0. You can calculate the subnet mask back to the CIDR notationâ€¦

255 = 8
255 = 8
192 = 2

So thatâ€™s a /18.

So in short, just â€śpickâ€ť a block size that matches all your networks and then figure out the subnet mask and/or CIDR notation.

The other quick method to look at it is like this:

172.16.0.0/24 = 1 network
172.16.0.0/23 = 2 networks
172.16.0.0/22 = 4 networks
172.16.0.0/21 = 8 networks
172.16.0.0/20 = 16 networks
172.16.0.0/19 = 32 networks
172.16.0.0/18 = 64 networks

Now you can see that 172.16.0.0/18 is the summary that will include all those networks that you wantâ€¦this is the quickest method.

Letâ€™s look at the second example:

192.168.0.0 / 24
192.168.1.0 / 24
192.168.2.0 / 24
192.168.3.0 / 24
192.168.4.0 / 24

Letâ€™s do the â€śblock sizeâ€ť method first. You can choose between 2,4,8,16,32,64 or 128. 2 and 4 are too small so weâ€™ll go for 8.

256 - 8 = 248 so the subnetmask will be 255.255.248.0

255 = 8 bits
255 = 8 bits
248 = 5 bits

So the CIDR notation is /21.

The other quick method to look at this is like this:

192.168.0.0/24 = 1 network
192.168.0.0/23 = 2 networks
192.168.0.0/22 = 4 networks
192.168.0.0/21 = 8 networks.

So weâ€™ll go for 192.168.0.0/21 as our summary. This is the quickest method to find the summary address.

Hope this helps! The only thing to be aware of is that your summaries include networks that you â€śdonâ€™t haveâ€ť.

Rene

1 Like

Rene,

â€śThe only thing to be aware of is that your summaries include networks that you â€śdonâ€™t haveâ€ť.â€ť

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.

Iâ€™m really grateful/thankful with your explanations. Thanks, thanks and thanks!!!

Hug

Hey Rene,
I was following you example but got confused on these:
172.1.4.0/25 10101100.00000001.00000100.00000000
172.1.128.0/25 10101100.00000001.10000000.00000000
172.1.5.0/24 10101100.00000001.00000101.00000000
172.1.6.0/24 10101100.00000001.00000110.00000000
172.1.7.0/24 10101100.00000001.00000111.00000000

Particularly the 2nd one 172.1.128.0/25. Following your example using the 3rd octet has a 128 in it, so how would I factor this in as the other octets are using the first 5 bits.

thanks
James

Hi James,

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, itâ€™s similar to how you solved the other two.

thanks
James

Ah I seeâ€¦they probably included it to throw you off guard

You can summarize 4,5,6 and 7 to 172.16.4.0 /22

Hi Rene

What are the disadvantages of using Route Summarization? Can you explain those with some examples pl.

Hi Lokesh,

There are a couple of potential issues.

1. You can blackhole traffic. For example letâ€™s say you have these 4 networks behind a router:

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.

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

2. 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 wonâ€™t be able to change metric (or BGP attributes) for a single prefix.

Rene

For #1, why will router receive traffic for 192.168.6.6 subnet, can you give me a topology or some conditions under which router is likely to receive traffic for other subnets unless it is designed in-correctly.

All addresses from 192.168.0.0 to 192.168.15.255 fall under the 192.168.0.0 /20 summary so if you advertise that summary in a routing protocol, other routers will use it.

192.168.6.6 falls within that range so if another router doesnâ€™t have a more specific route than the 192.168.0.0 /20 summary it will use it.

hi

ip range 126.89.120.0/22

how many sunbets will that cover and what ranges

Hi Aaron,

126.89.120.0 looks like this in binary:

01111110 01011001 01111000 00000000

Letâ€™s look at the third octet, since thatâ€™s where 22nd bit is:

120 = 01111110 01011001 01111000
121 = 01111110 01011001 01111001
122 = 01111110 01011001 01111010
123 = 01111110 01011001 01111011

So there it is, it matches 126.89.120.0, 126.89.121.0, 126.89.122.0 and 126.89.123.0

Rene

so will it cover
126.89.120.0/27
126.89.120.64/27
126.89.120.96.27

and so on ?

Yes, we are working with the third octet here so anything in the fourth octet is included.

so will it be good to advertise the summary route 126.89.120.0/22 to all eigrp neighnours to form relationships