i am practicing subnetting, then i got this question from the web.
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.
i am practicing subnetting, then i got this question from the web.
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.
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.