A network administrator has been given a network of 172.16.85.0/21 and wants to know the usable range of IP addresses on that subnet. Which of the following indicates the correct IP address range?
A. 172.16.80.0-172.16.85.254
B. 172.16.80.0-172.16.88.255
C. 172.16.80.1-172.16.87.254
D. 172.16.80.1 -172.16.85.255
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Everything aside, this question basically wants to know if you know the network ID .0 is unusable and the broadcast ID .255 is unusable. The only Usable range is C .1-.254
why am i getting the answer as 172.16.80.1-172.16.95.254? Im using cidr notation to get that answer but not understanding why its incorrect. if i have come up with the net id of 172.16.80.1 and i add 16 to the 3rd octet and subtract 1, i end up with 95.
I’m not sure where you’re getting the “add 16 to the third octet” part.
with a /21 on a class B address like this, that CIDR means 5 bits are being borrowed from the 8 bits in the 3rd octect. (so the way I do it) you take the value of those 5 bits (248) and subtract that from 256. That leaves you with 8, which are your block sizes. That basically means your next subnet for this class B IP would be 172.16.88.0
everything in between is the range of 172.16.85.0/21
that’s 2046 addresses. (which you can calculate by taking the number of bits left over in the subnet mask, in this case 11 and say 2 to the power of 11, which is 2048, then subtract 2 [for the net address and broadcast address] and you find the number of hosts in the subnet)
sorry if that was a poor or confusing explanation.