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.
10.0.8.0/20 10.0.16.0/22 10.0.21.0/23
– FrameHowitzer Jun 16 '23 at 14:09If 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