CCNA Self Study ICND - Chap 8

Daniel333

Hey everyone,


I just did the "Do I know this already" section for Chapter 8 and only got this single question wrong, and I've reread the sections related about 4 times so I was hoping some one could explain this or find a good link that I've not.

#2) Which of the following summarized subnets are valid routes according to the main purpose of CIDR?

a) 10.0.0.0 255.255.255.0
b) 10.1.0.0 255.255.0.0
c) 200.1.1.0 255.255.255.0
d) 200.1.0.0 255.255.0.0

The Book: D
Mine: B,C,D

My reasoning is aren't they all summarized? A is the only one that is not summarized isn't it?

thanks, as always, in advance, [/i]

markzab

I haven't brushed up yet on my summarization but if you'll notice, D is the only one where the subnet mask is behind the default mask. What I mean by that is, a network address starting at 200 would be considered a Class C, giving it a default mask of 255.255.255.0. In our example D has a mask of 255.255.0.0, which to me is saying that all subnets in the 3rd octet would be summarized back.

Ugh, I know this isn't coming out right. Ok, look at every other address there. The masks that they have are all at or past their default boundary. Only D's resides before it's default boundary, hense summarizing all of it's 3rd octet subnets.

Ok, if I'm right, cool...if not...

I'll apply my own signature below to this post.

Daniel333

Bam! Right on man. I see it now. It's the only answer where summarization is in play, you can't summarize to a larger subnet mask, would defeat the point.

Thanks!

markzab

Phew, eek'd that one out.