Most people learn subnetting as a pile of formulas: count the bits, subtract two, memorize the table. That works for exams, but it falls apart the moment you are standing in front of a real network with an IP like 10.0.5.130/27 and someone asks "what network is this on?" This article is the version I wish someone had given me: a single mental model that turns every subnetting question into arithmetic you can do in your head, plus the three shortcuts that make it fast. The one id...