Network addresses solve an impossible problem: how do billions of devices find each other without every router knowing about every device? The answer is hierarchy—and it's simpler than you'd think.
The subnet mask answers one question for every packet your device sends: can I reach them directly, or do I need a router? Here's how that split-second decision works.
Subnets answer the question every packet asks: do I belong here? CIDR notation draws that boundary with a single number—replacing the cleaver of class-based allocation with a laser.
Every packet your device sends asks: same subnet or router? Subnet calculation answers in microseconds. Learn the binary mechanics, then build speed with mental math that makes CIDR notation obvious.
When a device joins a network, it faces an impossible problem: it needs to communicate before it has an identity. Broadcast addresses solve this by letting devices shout into the void—and somehow, it works.
The Internet was two years from collapse when CIDR arrived. Here's how a 1993 emergency redesign bought IPv4 three extra decades—and what it reveals about building systems that survive their own success.
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