Summarization Blues...

Welp, it's official I wasted a day on summarization, and still don't get it. Here is another one i got wrong, on that topic. Anyone have a good tutorial on this sort of question laying around?

A bit is in the 8th place, and 56 doesn't even have anything in that place. So the only bits they share in common are the 2nd and 3rd bits. for 48, so D has the best summery as far as the address. but the mask is too high. Should be 224 not, 240. So I think B, C and E are the valid summaries. What am I not getting?

Incorrect.
Your answer was: D
The correct answer(s): A

Which of the following summarized subnets is not a valid summary that includes subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0?

A.) 10.1.55.0 255.255.255.0
B.) 10.1.0.0 255.255.0.0
C.) 10.0.0.0 255.0.0.0
D.) 10.1.48.0 255.255.240.0
E.) 10.1.32.0 255.255.224.0

Cisco Press Book Reference: PDF Page - 276 QID: 103 Cisco Press CCNA ICND

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Comments

NightShade1

Click on the link on my signature and there is a summarization tutorial in it....

Anyways i will give you a fast explanation why is A

A is the correct asnwer because its just common sense look

10.1.55.0 255.255.255.0 look at the mask....255.255.255.0 What does this mean? well it means it will check ALL the bits of the 3 first octets.....which means it will check that 10.1.55 must be the same always....

so it will check 10 that must be equal... 1 that Must be equal and 55 that must be equal.... it doesnt check the 0 so it can vary... Because the netmask of 255.255.255.0 doesnt allow him to vary.

if you see 10.1.55.0, 10.1.56.0, and 10.1.57.0, the third octet is varying.... so is not possible that 10.1.55.0 255.255.255.0 include those networks...

B is incorrect becasue it includes those networks...
10.1.0.0 255.255.0.0 it checks 10 and 1 but doesnt check 0 and the other 0 so the third octect and the last octect can vary to any number so it include it...

C is incorrect for the same reason of B

D is incorrect 10.1.48.0 255.255.240.0 now if you see the increment of the networks are of 16...
so we get
10.1.0.0
10.1.16.0
10.1.32.0
10.1.48.0
10.1.64.0

As you see 10.1.55.0, 10.1.56.0, and 10.1.57.0 are included in the netwrok 10.1.48.0 255.255.240.0

E is the same thing like with D
10.1.32.0 255.255.224.0 means an increment of 32 so we got

10.1.0.0
10.1.32.0
10.1.64.0

As you see 10.1.55.0, 10.1.56.0, and 10.1.57.0 are included in the netwrok 10.1.32.0 255.255.224.0

How do i know the increment? well looking at the mask...
224=32
240=16
192=64
blah blah blah

ignign0kt

Well it's definitely not the fastest way, but the easiest way to understand is to look at all the addresses in binary:

Here the 3rd octets of the addresses in the question:

00110111
00111000
00111001

So now what we want to do, is look at the binary of the addresses that we have to choose from (again third octets, since that's what we are comparing)

A 00110111
B 00000000
C 00000000
D 00110000
E 00100000

We check to see (from left to right), if a 1 matches up with all of the above addresses in the same place For example, E can be a route summary of all the addresses because all 3 addresses have a 1 in the decimal 32 position. D can be a route summary because all 3 addresses have 1's in their decimal 32 and 16 positions. We do the same for C and B, and they match. But with A, it only matches the 1st address and no the others because they don't have 1's in the same position.

So..it's basically determining which have the most in common in terms of the number of bits from left to right.

Hope that helps a little ;\

malweth

Also ensure that you're not misinterpreting the question.

...summarized subnets is not a valid summary...

It could just be coincidence, but you chose the "best" summary (most narrow), not the one that doesn't work.

On a question like this, you should be able to see:

10.1.55.0 255.255.255.0

and immediately realize that this is not a summary address of 10.1.56.0/24. No math involved (unless it's a choose-many type of question)!

mzgavc

Just a word of advice,

Doing this question without reading the answers given by everyone else, I picked the same answer as you... then I read everyone elses remarks.

I don't know if its the same thing but I didn't read the question properly:

Which of the following summarized subnets is not a valid summary that includes subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0?

Once I saw the "not"..... there was a resounding forehead slap.