Every bit moved from host to network halves the addresses — a /24 has 254 usable.
2^(host bits) minus network and broadcast. /25 → 126, /26 → 62, /30 → 2 (the classic point-to-point link).
The minus-two for network and broadcast addresses stops applying at the small end: a /31 has two usable addresses, not zero, because RFC 3021 redefined it for point-to-point links. And in IPv6 there is no broadcast address, so the arithmetic is different from the start.
At 26 the rule says 64 and the exact answer is 62 — 3.2% high. It holds to within 5% up to 26.6, then drifts.
Two addresses in every subnet are spoken for - the network itself and the broadcast - so the halving is only true while the subnet is big enough for two to be a rounding error. On a /30, the size used for point-to-point links, the rule promises four addresses and you get two.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Subnet Calculator →Every bit moved from host to network halves the addresses — a /24 has 254 usable. 2^(host bits) minus network and broadcast.