Binary 00000000  =  Decimal 0

32-bit binary 

= 

Enter an IP and prefix. The network bits (mask = 1) are kept; the host bits (mask = 0) become 0 — that gives the network number.

Given a network and a requirement, find the prefix: 2ˣ − 2 ≥ hosts needed; 2ˢ ≥ subnets needed.

List each subnet's host need. The planner sizes each (largest first) and lays them out from the base block with no overlap.

Split a network into equal subnets and read off any subnet's network, usable range, and broadcast — e.g. “first usable in the 3rd subnet?”