When I first took the CCNA, I used a shortcut to get subnet masks worked out correctly. This was important, since I didn’t know cold what a /14 was, a /22 was, and so on. I got away by knowing only the classful subnet breaks (/8, /16, /24, /32 — hereafter referred to without the slashes) along with the knowledge that subnets all work (in binary) from left to right with the progression 128, 192, 224, 240, 248, 252, 254, then 255. Rather, I knew those pieces, and I knew what came in the middle, and that was enough to get me through the binary and decimal conversions. Let me explain.
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