Full Version: how to tackle this route summarization qs?
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what is the most appropriate summarization for these routes?



well i think i got it. the answer is (b).

(12-14-2010, 03:31 PM)kelvinwgr link Wrote:well i think i got it. the answer is (b).

Same answer I got and Im confident its correct.
My method:

    8      +    8    +6(common bits)      = /22
    10      .      0    .    0        .    0      /22
So the first two octets are common between all 4 and are 8 bits each so 8+8 = 16
Also the 3rd octet has 6 bits in common with all 4 addresses as shown above, 16+6 = 22
so is the answer


The faster way with no Binary Math is: This will blow your mind how easy it is.

1) Find the Block size to cover all of the networks in the 3rd octet= Networks 0,1,2,3 answer is 4 block size.
2) Now count on your fingers backwards: 128,64,32,16,8,4,  Should have 6 fingers sticking up.
3) How many common bits in the first 2 octets, 8+8= 16, Now add your like bits from your finger counting 16+6=22
4) Answer is
5) Again, no binary math and it all works from your 128,64,32,16,8,4,2,1 chart

For example: Summarize these networks?

1) What block size covers the all of the 3rd octets? Answer is 16
2) Count backwards on your fingers: 128,64,32,16
3) 4 fingers are up, added to the first 16bits. 8+8+4=20
4) Block size started at 16 so the answer is


@ Hondabuff

Could you use your method on

Do you have to count block from subnet boundary?



block that covers meaning > than not = to.



Could you use your method on

10.1.{0-8 not enough because of the 9, 0-16 will cover all of  0-2-9-16} .0 All routes from 1-15 will be included because of the overage.
Technically it would be considered a poorly designed network.

0-8, 0-16, 0-32, looks like all we need is 16 places to cover them all. We will be wasting places because the is in the next block of 8.
So a summarized network of A block size of 16 is a 240 mask.
The 10.1 make up 16 bits allready and 240 mask uses 4 bits. 16+4=/20