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?
For those wondering :
16 (classful subnet - 255.255.0.0)
21 ( classless subnet - 255.255.248.0) 5 bits borrowed from host portion
count in bit format to get your magic number ( block size) 128,192,224,240,248 = 5 bits which equals 8 block size, so you count in 8's to for your networks
in this case .80-87 (88 is the last IP which is broadcast so you cannot use that)
Keep going you can do it!
Since we know .88 is the next subnet, then the broadcast is 87.255, then the last usable IP is 87.254. I had a hard time with subnetting because I had to learn when they are offering me the target IP or the network ID. You do different things depending.
For CIDR subnetting, you need to convert the IP into binary. The number after the / is the number if bits reserved for the network.
172.16.85.0 in binary is 10101100.0010000.01010101.00000000. To find the network address, we keep the first 21 bits and set all bits from the 22nd on to 0, giving us 10101100.00010000.01010000.00000000, or 172.16.80.0, which is our network address.
To get the broadcast address, we flip the 22nd bit on to 1, giving us 10101100.00010000.01010111.11111111, or 172.16.87.255.
To get the usable range, we add one to the network address and subtract one from the broadcast address, giving us 172.16.85.1 to 172.16.87.254.
find your block size. A /21 will have a block size of 8 due to having to borrow 5 bits. Divide your block size by the octet you borrowed from (3rd octet) and do not worry about remainders. 8 will do into 84 at least 10 times. Multiply that answer by the block size and you get 80. This is the start of your subnet. Your block size is 8 so your next sub net is 88. The first IP (80.0) is the network address so no host can use it and the last (broadcast) is 87.255 and can not be used by a host also. It is x.x.80.1-x.x.87.254
The last is: 87.255, this is also the broadcast.
The last usable is: 87.254, this can be assigned to a host.
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