Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Community Member

Need help with Summarization

Hello All,

There was a need to summarize the following addresses

3.3.8.3 - 3.3.11.3/24

and also

4.4.12.4 - 4.4.15.4/24.

My answers to the two summaries were 3.3.4.0/22 and 4.4.4.0/22

But unfortunately the book says the answer is 3.3.8.0 and 4.4.12.0/22

Will greatly appreciate thorogh explanation as to why this was so.

Thank you all

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Need help with Summarization

Think of it this way:

3.3.8.0/24

3.3.9.0/24

3.3.10.0/24

3.3.11.0/24

How many subnets do you have? 4

How many times to the power of 2 can I use to get 4? 2

Subtract that 2 from 24 = 22

Take the first subnet and add the 22 as your new mask

3.3.8.0/22

Another example would be:

172.16.0.0/24

172.17.0.0/24

172.18.0.0/24

172.19.0.0/24

172.20.0.0/24

172.21.0.0/24

172.22.0.0/24

172.23.0.0/24

How many subnets? 8

How many powers of 2? 3

Subtract 3 from mask: 24-3 21

Answer: 172.16.0.0/21

It does get weirder when you cross boundaries. For instance:

Here's a good document for you to look at:

http://subnettingmadeeasy.blogspot.com/2007/11/router-summarization.html

HTH

--John

HTH, John *** Please rate all useful posts ***
8 REPLIES
Hall of Fame Super Blue

Re: Need help with Summarization

You are trying to summarise 4 networks in each case ie.

3.3.8

3.3.9

3.3.10

3.3.11

/22 = 255.255.252.0

252 gives you 4 networks ie.

256 - 252 = 4

so 3.3.4.0/22 =

3.3.4

3.3.5

3.3.6

3.3.7

All the above applies to 4.4.12 - 4.4.15

Does this makes sense ?

Jon

Community Member

Re: Need help with Summarization

Hi Jon,

I'm grateful for the reply but just wished your response could address my query more precisely?. All i want to know is if i am correct or the book author is, and if so with explanation as to how

3.3.8.3/24

3.3.9.3/24

3.3.10.3/24

3.3.11.3/24

could become 3.3.8.0/22

And how

4.4.12.4/24

4.4.13.4/24

4.4.14.4/24

4.4.15.4/24

could be become 4.4.12.0/22?

Hence my answer was 3.3.4.0/22 and 4.4.4.0/22 respectfully.

Any further clarification shall be appreciated.

Regards

Hall of Fame Super Blue

Re: Need help with Summarization

Oh sorry, i thought i had explained, my fault.

The book is correct.

Your answer is

3.3.4.0 255.255.252.0

255.255 match the 3.3 and they won't change.

4 matches the 252 and this will change.

252 means you can have 4 different values ie.

256 - 252 = 4

So starting with

3.3.4.0 that is 1

3.3.5.0 that is 2

3.3.6.0 that is 3

3.3.7.0 that is 4

so your answer 3.3.4.0/22 covers the networks

3.3.4.0/24

3.3.5.0/24

3.3.6.0/24

3.3.7.0/24

Now you know with 252 you can have 4 values so

3.3.8.0 is 1

3.3.9.0 is 2

3.3.10.0 is 3

3.3.11.0 is 4

which is the exact range the question asked for. So the book is correct.

Jon

Community Member

Re: Need help with Summarization

Hello Jon,

Most grateful again for your time to explain. Nevertheless, if you understood my last query that means i am correct while the book is wrong?.

Here again is what i came up with

"Hence my answer was 3.3.4.0/22 and 4.4.4.0/22"

Which is exactly the same as yours.

Thanks

Hall of Fame Super Blue

Re: Need help with Summarization

Sorry, i'm making a real mess of explaining this - perhaps someone else could step in ?

Note i am dealing with 3.3.x.x networks but it applies to 4.4.x.x networks as well.

Your original query was

There was a need to summarize the following addresses

3.3.8.3 - 3.3.11.3/24

Now i'm assuming you mean you need to summarize

3.3.8.0/24

3.3.9.0/24

3.3.10.0/24

3.3.11.0/24

Now my previous posts were trying to show that your answer

3.3.4.0/22 covers the following networks

3.3.4.0/24

3.3.5.0/24

3.3.6.0/24

3.3.7.0/24

which is not the networks you are asked to summarise. So the actual answer is 3.3.8.0/22 which covers

3.3.8.0/24

3.3.9.0/24

3.3.10.0/24

3.3.11.0/24

It could be me, am i missing something obvious ?

Jon

Community Member

Re: Need help with Summarization

Compare your answer with a typical /30 we often use for a serial connection by dividing a class C network: for example 3.3.3.8 / 30.

When you use this network, you have a network address (3.3.3.8), two usable addresses (.9 and .10) and a broadcast address (.11). In this example you have divided the range of available addresses (256) into 64 smaller networks with each 4 addresses by using 6 bits of the fourth octet to define new networks (hence /30 (24+6).

Your case at hand does the same but for the third octet. You divide 3.3.0.0 /16 into 64 networks which include each 4 networks. You use 6 bits of the third octet (hence /22 (16+6) to create 64 networks with the following network addresses: 3.3.0.0, 3.3.4.0,3.3.8.0, 3.3.12.0 (...), 3.3.252.0

Your solution uses .4 as network address for the new subnet, which would mean (since you have a /22 mask) your network includes 3.3.4.0, 3.3.5.0, 3.3.6.0 and 3.3.7.0. Since you need to summarize 8 thru 11, your first network must be .8

The reference above is excellent, I also liked

http://www.coxpc.com/content/3com_Chuck_Semeria.htm, for the included tasks.

HTH, Thomas

Community Member

Re: Need help with Summarization

Re: Need help with Summarization

Think of it this way:

3.3.8.0/24

3.3.9.0/24

3.3.10.0/24

3.3.11.0/24

How many subnets do you have? 4

How many times to the power of 2 can I use to get 4? 2

Subtract that 2 from 24 = 22

Take the first subnet and add the 22 as your new mask

3.3.8.0/22

Another example would be:

172.16.0.0/24

172.17.0.0/24

172.18.0.0/24

172.19.0.0/24

172.20.0.0/24

172.21.0.0/24

172.22.0.0/24

172.23.0.0/24

How many subnets? 8

How many powers of 2? 3

Subtract 3 from mask: 24-3 21

Answer: 172.16.0.0/21

It does get weirder when you cross boundaries. For instance:

Here's a good document for you to look at:

http://subnettingmadeeasy.blogspot.com/2007/11/router-summarization.html

HTH

--John

HTH, John *** Please rate all useful posts ***
142
Views
0
Helpful
8
Replies
CreatePlease to create content