What this information does is given you three things: The number of available hosts, the subnet range of available hosts once you find the network number, and also the available number of subnets.Īs seen in the chart, the subnet bits are counted from where the assigned mask stops, and grows exponentially by the power of two (fancy way of saying they double) as they go further into the “Host Bits” available. So you can count backwards from 32 and get the above values, but again I strongly recommend you write a chart out to reference quickly for subnetting on the dry erase board your provided, even if it’s just a tiny table in the corner of the sheet. This is a way of saying that we can start at a /32, and find how many bits we have entirely, then apply the formulas -2 for subnet / broadcast address as such:īeyond that, I will let you continue to double the number, minus two bits for network / broadcast addresses. “From the right rearmost we discover our hosts” Given that we already know from CCNA studies, that /24 indicates that the first 3 octets are spoken for, and we can borrow bits from the host bits we are provided to create subnets, and any left over bits are our available host bits (minus 2 for network and broadcast address of course). You may be presented with a question like “A carrier provides an IP Subnet of 150.200.100.0/24, and you must make x amount of subnets that provides for the most amount of users” That being said, I stole the above graphic from Keith Bogart’s CCNA subnetting class (with his express permission), and wanted to post this up not only for my but anyone elses review how to come up with answers fast on exam day for questions regarding different subnets. While going through some practice tests, I realized how stagnant my subnetting had become, and the only thing I remember was writing everything out in binary which was a time killer both on the practice exam in definitely will be in the class room. Let me preface this by advising you write some sort of version of this chart on your dry erase board before beginning the exam, so you are not scrambling to remember it while the clock is ticking down during the exam, and if you don’t understand this chart read about it below until you do!
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