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I am not understanding how to determine the first network address or "first IP address of block" per Cisco Press documentation when summarizing routes/subnets. I am clear on how to get the summarized prefix, but not the first network address.

Here is an example:

172.16.12.0/24
172.16.13.0/24
172.16.14.0/24
172.16.15.0/24

This is summarized specifically as: 172.16.12.0/22. Simple, convert to binary and start left-most and count number of common bits and that is the prefix mask

This also makes sense to me because the first network address in the summary is the "base" of all the addresses taken into account: 172.16.12.0, not 172.16.0.0

Now take this for example:

172.16.10.0/24
172.16.20.0/24
172.16.30.0/24
172.16.40.0/24
172.16.50.0/24

The answer is reportedly according to one source: 176.16.0.0/18

This is my confusion. Does 176.16.0.0 represent "the first IP address in the block"? Why is it not 172.16.10.0 instead, as this is what I would deem to be the answer as it limits the amount of network by eliminating any networks by not allowing ones between 172.16.0.0 - 172.16.9.255. I understand the /18 part of it.

Am I mistaken? How do you know the first IP address of the block when summarizing?

TIA

humbleStrength
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  • This two-part answer has a section that explains it all. – Ron Maupin Dec 11 '23 at 14:45
  • @RonMaupin I went through that answer but there is no solution described that applies to my question that I am discerning. The section on the First Usable IPv4 Network Host Address is the closest topic, but it does not cover the first IP address in the block when summarizing routes/networks as in my example. The 172.16.0.0\18 and not 172.16.10.0\18, the former allows for more networks that it should not. How does the answer come to 172.16.0.0\18 in my second example? – humbleStrength Dec 19 '23 at 15:56
  • I guess you really do not understand the answer. There is a section called Largest Common Network for Addresses (Address Aggregation) in Part 2 that explains exactly how to do that. – Ron Maupin Dec 19 '23 at 16:09
  • My apologies. I did not see that section as that would be very apparent to me. I stopped in part 2 at the Find A Particular Subnet section. Please pardon my premature comment and thank you for the guidance. – humbleStrength Dec 19 '23 at 17:39
  • I guess it is because my answer is so large to try to include almost all aspects of IPv4 addressing and subnetting. In reality, you need to understand all the previous sections to fully understand a particular section. – Ron Maupin Dec 21 '23 at 02:36

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