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256-240 = 16 - so you have the subnets going up in increments of 16 starting with zero. Each subnet will need to have a subnet and a broadcast number so this leaves 14 usable host numbers per subnet. The subnets start at 0,16,32,48, 64, 80….224, 240.We stop at 240 because this is the last number of the above subnet mask. Subnet First host Last host Broadcast 1. 172.16.5.0 1 14 15 2. 172.16.5.16 17 30 31 3. 172.16.5.32 33 62 63 4. 172.16.5.64* 65 78 79 Last 172.16.255.240 241 254 255 Looking at the 172.16.5.68 host address, we are clearly looking to find the host on the fourth octet because this is where the non-zero mask is (255.255.255.240). We just keep adding 16 until we findthe range that the host number 68 is in. In this case 172.16.5.64 as indicated by the asterisk*. The Super Subnetting Chart™ will also provide the answers for you very quickly. To get to the 240 mask we tick down four subnet numbers and then tick four along the top to get the subnet increment i.e. 16). I know 172 is a class C network but this doesn’tmatter for subnetting. We just work on the octet the subnet is happening in. 17
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