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VLSM
Cutting one block of addresses into subnets of different sizes, each as big as its network needs.
A /n subnet holds 2^(32 − n) addresses, two of them reserved, so 50 PCs need a /26 (62), 20 phones a /27 (30) and a few printers a /28 (14). Carve the biggest first, each starting on a multiple of its own size, and they never overlap. A link between two routers takes a /30 (two hosts) or a /31 (RFC 3021: both addresses are hosts, no network or broadcast address). Subnets carved from one block can later be summarised into one route.