What is the fastest way to find the network address without binary math?
Given something like 172.16.37.99/27, converting to binary takes me minutes. Is there a mental shortcut that works under exam time pressure?
1 Answer
Use the block size method. Find the "interesting octet" — the one where the mask isn't 255 or 0. /27 → mask 255.255.255.224, interesting octet is the 4th. Block size = 256 − 224 = 32, so subnets start every 32: .0, .32, .64, .96, .128… Your address's 99 falls in the .96 block → network 172.16.37.96, broadcast is the next block minus one → .127, hosts .97–.126. Three subtractions, no binary. Practice until block sizes are reflexes: 128, 64, 32, 16, 8, 4 correspond to masks .128, .192, .224, .240, .248, .252. The subnetting drills chapter (Module 8) generates endless variations of exactly this, and the free subnet calculator at /tools/subnet-calculator will check your answers.