Subnet Count Calculator
Calculate how many subnets can be created from a network with a given prefix length.
About Subnet Count Calculator
Subnetting works by borrowing bits from the host portion of an address and reassigning them to the network portion. Borrowing n bits multiplies the number of subnets by 2^n while dividing the hosts available per subnet accordingly. The new prefix length is simply the original prefix plus the number of borrowed bits.
Formula
Subnets from borrowed bits
subnets = 2^borrowed_bits
- n
- Number of bits borrowed from the host portion
Worked example
With the default values below, Subnet Count Calculator returns Subnets Created of 8. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Original Prefixaccepts 0–30 /
- 24 /
- Bits Borrowedaccepts 0–30
- 3
Results
- Subnets Created
- 8
- New Prefix
- /27
- Hosts per Subnet
- 30
Step by step
- 1Values used
- 2Subnets from borrowed bits
- 3Subnets Created= 8
- 4New Prefix= /27
- 5Hosts per Subnet= 30
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
What does 'borrowing bits' mean?
It means taking bits that were previously part of the host identifier and using them instead to identify individual subnets, which increases the prefix length by that many bits.
How many subnets does borrowing 3 bits create?
2^3 = 8 subnets. Each additional borrowed bit doubles the subnet count while halving the hosts available in each subnet.
Is there a tradeoff when borrowing more bits?
Yes — every bit borrowed for subnets is a bit no longer available for hosts, so you gain more, smaller subnets at the cost of fewer usable addresses per subnet.