Summarization and Default mask

[tsunamoro]

Hi all.

On page 115 it's said:

"These (subnet mask for Classes A, B and C) default masks cannot change. [...] For a Class A network, [...] it must read 255.0.0.0 at a minimum. [...] A Class B address (should be "mask"?) must start with 255.255.0.0, and a Class C has to start with 255.255.255.0."

Now, when some summarization examples are given, on pages 147-148, in order to summarize the networks from 192.168.16.0 to 192.168.31.0 a subnet mask of 255.255.240.0 is used. This is, in my opinion, in contraddition to what is said on page 115.
I'm confused.
Would somebody, or maybe Todd, make this point clear to me?

Thank you very much.

[dbeare]

I see what you are saying, and I can understand why you are confused, but let me assure you, you can do what is talked about with the summarization. The default mask for each of those networks is as follows:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0

That is for networks that do not use subnets, they just use the default range. Now that we are able to subnet, we can use different subnets to break up IP ranges and only use the number of IP addresses that we actually need, rather than having to use an entire range of IP addresses. This allows us to do exactly what was talked about later when you mention network summarization. With subnetting, we now can summarize a network of 192.168.16.0/24 through 192.168.31.0/24 by using 255.255.240.0 as the subnet mask and creating at 192.168.16.0/20 network.

Clear as mud? Just keep reading the book over, and hopefully it will click for you. Just remember the difference between the default subnet masks and masks created when you do subnetting. Hope this helped a little!