IPv4 Subnetting Tricks
Fast, dependable subnet-boundary methods for timed questions and real troubleshooting.
Magic-number method
Boundary trick
Block size = 256 minus the interesting mask octet.
List multiples of the block. The address belongs to the interval whose lower multiple is at or below it.
Network = lower boundary; broadcast = next boundary minus one.
Exam traps
A lower prefix number means a larger network.
Longest-prefix match is considered before administrative distance.
Always calculate the broadcast boundary before choosing the usable range.
Why the method works
A subnet mask divides an address into fixed network bits and variable host bits. The interesting octet is where that boundary falls.
The block size tells you how often subnet networks repeat in that octet. For a /27, 256 − 224 = 32, so networks begin at 0, 32, 64, 96, and so on.
Example: 192.168.10.77/27 falls in the 64–95 block. The network is .64, broadcast is .95, and usable hosts are .65 through .94.
Choose a prefix from a host requirement
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