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CIDR and address space

hello everyone,

it is long time and i think that CIDR is just a routing method that allows routers reduce routing table size.

tonight, i was reading a book mentioning that CIDR saves address space.

This makes sense of what follows:

  • assume your organization has been assigned two class C ranges and
  • CIDR may help reducing tables by advertising both as /23 []

my two questions are:

  • Is a broadcast address?
  • Is a network address?

if so, then we have not saved address space.

Actually, i have found many misundestandings regarding network concepts in this book and other books of the same publisher, but i still need to understand this in depth.

BTW, this is not a cisco press book

thank you in advance


Everyone's tags (2)

Re: CIDR and address space

Either it's a misquote, or the book has a typo, as it probably meant to read "CIDR may help reducing tables by advertising both as /23 []". The answers are "no" and "yes" to your two questions, respectively.

The address savings through CIDR can be seen more easily with the help of online tools such as

On older IOS, you may need to configure "ip subnet-zero" and/or "ip classless" to allow zero subnets and CIDR.


Re: CIDR and address space

hello yjdabear,

first of all, i worte /23 and obviously i meant /23. this is the correct summary address.

i cannot understand your answer. NO and YES respectively. How this ?

if is not a broadcast address, then how be a network address ?

network addresses always preceded by broadcast addresses except the first subnet.

let me re-prashe my question

how many host bit in the new netowrk ?

or let me say is CIDR just a routing method .. just a summarization .. or it really affects host/network portions.

finally, let IP addresses be and .. this make sense of class C addresses.

i ate a lot before writing this question so there is many mistakes in calculations

hahaha.. actually it should be "There are many mistakes" not "there is"

thank you



Re: CIDR and address space

here is the answer

Clare Gough - "CCNP BSCI exam certification guide" author has mentioned in her book the following:

Although eight Class C addresses are provided to the organization, they are identified to the Internet as one address, with a prefix of /21, which is the subnet mask of

The organization does not have to use the addresses as Class C addresses. In accordance with the original rules, the organization can use the right-most zeroed bits however it deems approperiate.

3rd Ed. page 54.

she was talking about /24 to /24

My conclusion:

Summarization is just a routing trick, but not a supernetting.

CIDR is a routing trick and a supernetting, which means that the right most zeroed bits are under your control. They are for host by definition, but you still can use them as classful, or subnet them as you wish.

This sounds logical and answers the question: Why bother studying both summarization and CIDR ?

thank you all,

Mohammad Nasim

CCNA Instructor

Message was edited by: Mohammad Nasim

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