Host Bits Required Calculator

Work out host bits required instantly with clear inputs, formula shown and shareable results.

About Host Bits Required Calculator

Host bits are the inverse of prefix length. To address H interfaces you need 2^h >= H + 2 when the network and broadcast addresses must be reserved, so h = ceil(log2(H + 2)) and the prefix is 32 - h. The spare figure shows how much of the next power of two is left over, which is the growth room the rounding gives you for free.

Formula

Host bits

h = ceil(log2(hosts + reserved)); prefix = 32 - h; usable = 2^h - reserved

Worked example

With the default values below, Host Bits Required Calculator returns Host bits required of 9. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts to addressminimum 1: 500
Reserve network and broadcastone of Yes - normal subnet, No - /31 or /32 special case: Yes - normal subnet

Results

Host bits required: 9 bits
Resulting prefix: 23 /n
Usable hosts in that prefix: 510
Spare capacity: 10

Step by step

1Values used
2Host bits
3Host bits required= 9 bits
4Resulting prefix= 23 /n
5Usable hosts in that prefix= 510
6Spare capacity= 10

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does 510 hosts need a /23?

510 plus the two reserved addresses is 512, which is 2^9, so nine host bits are required and the prefix is 32 - 9 = 23.

Does the answer change for IPv6?

Conceptually no, but IPv6 LANs are always /64 regardless of host count, so host-bit arithmetic is only used for IPv4 and for point-to-point IPv6 links.