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I am a student and I came accross this question:

An ABR in OSPF has in its range networks 10.0.8.9/24 to 10.0.21.0/24. how can it summarise the networks? not necessarily in one prefix.

What we ve learned about route aggregation is that I should convert to binary, count the number of similar bits between addresses and use that number as a prefix.

But. There is an example in the book Computer Networking: A Top-Down Approach, figure 4.21, that aggregates the addresses 200.23.16.0​, 200.23.18.0, 200.23.20.0 to 200.23.16.0/20

The way I have learnt to do this I would put /21. An online calculator also puts /21.

Additionally, trying to clear this up I read that there is a requirement for all remaining bits to be zeroes? That does not make much sense to me but it means that the prefix should be /23?

200.23.16.0 - 11001000.00010111.00010000.00000000

200.23.18.0 - 11001000.00010111.00010010.00000000

200.23.20.0 - 11001000.00010111.00010100.00000000

I am a bit confused about all these conflicting opinions on the same set of addresses and I am not sure which method to use to answer my question. If someone could explain which one is valid or how to chose between them in each case it would be very helpful.

riverwastaken
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  • I don't have that book in front of me, so I can't see if you're missing something from the original example. But I would also agree on a /21 based on your question. When they say remaining bits they mean all the (11) bits after the 21 bits. – Ron Trunk Jun 16 '23 at 13:05
  • This two-part answer has a section that covers this called Largest Common Network for Addresses (Address Aggregation). In any case, we cannot answer for what a book or teacher tells you (they are often wrong), which is why all {education, certification, or homework_" questions are explicitly off-topic here. – Ron Maupin Jun 16 '23 at 13:22
  • Speaking from a practical level, you cannot summarize networks 10.0.8.9/24 AND 10.0.21.0/24 by themselves unless you actually mean networks 10.0.8.9/24 TO 10.0.21.0/24 (which is what was written but sometimes people make mistakes). If you actually want to use that whole range of IP addresses then the most efficient summarization might be:

    10.0.8.0/20 10.0.16.0/22 10.0.21.0/23

     – FrameHowitzer Jun 16 '23 at 14:09
  • @FrameHowitzer i get 10.0.8.0/20 and 10.0.21.0/23, could you maybe explain how and why we include 10.0.16.0/22? thanks a lot (and yes i am talking about the range) – riverwastaken Jun 18 '23 at 13:03
  • 10.0.8.0/20 includes from 10.0.8.0 to 10.0.15.255 and 10.0.16.0/22 includes 10.0.16.0 to 10.0.19.255

    If you don't include 10.0.16.0/22 then you will be missing a huge chunk of IP addresses in the middle of the range you wanted to use.

     – FrameHowitzer Jun 18 '23 at 19:31

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