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i am practicing subnetting, then i got this question from the web. enter image description here

then the answer showed up as 172.16.0.0/12, i am puzzled right now how did that happen. please if anybody out there can explain to me how. thank you so much.

theweebokid
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1 Answers1

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Let's ignore 172.22.0.0/16 and 172.23.0.0/16, since there are between 172.21.0.0 and 172.24.0.0, any summary that include the .21 and .24 will also include .22 and .23.

Converting the 2 addresses in binary we got:

172.21.0.0 == 10101100.00010101.00000000.00000000
172.24.0.0 == 10101100.00011000.00000000.00000000

Now we see that only the first 12 bits are common to the 2 addresses, so the summary network is a /12

the network address is given by those 12 first bits, followed by 0 to complement to 32 bits.

So we convert back 10101100.00010000.0000000000.000000000 to decimal notation and we get 172.16.0.0

conclusion: the summary network is 172.16.0.0/12

See this excellent answer to the question How do you calculate the prefix, network, subnet, and host numbers? for more details.

JFL
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