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
172.16.0.0\18
and not172.16.10.0\18
, the former allows for more networks that it should not. How does the answer come to172.16.0.0\18
in my second example? – humbleStrength Dec 19 '23 at 15:56