Summarization - please help

[echoreply]

on pg. 149 there is a figure with ip addresses of:

172.1.4.0/25
172.1.5.0/24
172.1.6.0/24
172.1.7.0/24
172.1.4.128/25

The answer explains that the 3rd octet has a block size of 4. It says that the mask is 255.255.252.0 and that is a block size of 4.

I thought a /24 is 255.255.255.0 w/ block size of 256 (256-0=0)
and /25 is 255.255.255.128 w/ a block size of 128 (256-128)

Please help me understand how the 3rd octet is a 252 mask.

[lildeezul]

Yes... but we are talking about route summarization.
you will understand this better once you get to routing protocols, especially EIGRP and OSPF...

todd list the network....

172.1.4.0/25
172.1.5.0/24
172.1.6.0/24
172.1.7.0/24


now the objective is to summarize these 4 networks into 1, so that the routing table wont be so congested.
again you will learn in routing ospf and eigrp.

but this is what you do...
you find where all 4 networks have in common.... (convert to binary if you want)
you can see that all four have 172.1 in comming. and the third octect is where it starts to variate and differenciate. (spelling) so we are going to look at the third octect and we see that the number range from 4-7....

now think....!!!! what mask will give us a smallest block size that will consist of numbers 4-7... hmm i am thinkin a block size of four... Correct??

yes.. So a block size of four in the third octect is 255.255.252.0 ..

do you get it????

hopefully this helps..

the goal is to see where the networks are in comming, and where they start to differ. in the third octect.....
then you find the range in that third octect. which is 4-7...
then ask youself, what is the smallest block size that will accommadate these number..
a block size of two (255.255.254.0) wont work becuase its to little... and block size of 8,16,32,64,128 would work, but your trying to find the smallest block size that will work..

which is 4..

hope this helps

[echoreply]

Yes it does... Thanks for your help!