CIDR Calculator
Carve a block — how many children a parent prefix surrenders, what each child holds, and where the first and last ones land.
CIDR Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- One parent, one uniform child size — no mixed plan
- Aligns the base to the parent edge and says so
In short: Carving 10.0.0.0/16 into /24s borrows 8 bits and yields 256 subnets of 254 usable hosts each — the first child is 10.0.0.0/24, the last is 10.0.255.0/24, and the step between neighbors is 256 addresses. Subnetting is this one exponential: every borrowed bit doubles the count and halves each child.
Formula
children = 2child−parent · each = 232−child − 2 hosts
A carve is a trade of depth for count. Borrowing s bits splits the parent into 2 to the s children — each one 2 to the s times smaller. The parent's edge is the only honest starting line: if the address you type sits inside the block rather than on its edge, the page aligns it down and says so, because a carve that starts mid-block would slice addresses that were never yours to slice.
Worked Example
- Type the base address of the block you own.
- Enter its current prefix.
- Enter the child prefix you want to carve.
- Read the count, the per-child ledger and the edges.
Defaults: 10.0.0.0/16 carved to /24 → 256 children of 254 hosts. Drive the child to /26 and the same parent yields 1,024 subnets of 62 hosts — the 8-bit borrow becomes 10.
Strengths & Limits Of This Model
Where this engine is strong
- Borrowed bits and step printed, not just the count
- Supernet attempts refused by name
Where it stops
- No per-child naming or VLAN tagging — arithmetic only
Practical Use Cases
VLAN planning
one /16 into per-floor /24s
Cloud VPC design
carve to the tier size you need
Address audits
price what a borrow costs in hosts
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
CIDR Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many subnets come out of a carve?
Two to the power of the borrowed bits — child prefix minus parent prefix. Going from /16 to /24 borrows 8 bits and yields 256 children; from /16 to /26 borrows 10 and yields 1,024. Each borrowed bit doubles the count and halves every child.
How many hosts survive in each child?
The same law as always: 2 to the (32 minus child prefix), minus the network and broadcast addresses. A /24 child holds 254, a /26 holds 62, a /30 holds 2. The parent's own reserved pair is inherited by every child — each one carries its own network and broadcast address.
Why must the carve start on the block edge?
Because the parent prefix is a promise: everything inside it routes as one block. A carve computed from 10.0.1.5/16 would produce children that cross the /24 boundaries other networks already assume. The page aligns the base down to the parent's network address and prints the alignment instead of silently carving from mid-air.
What if the child prefix is shorter than the parent?
That is not a carve — it is a supernet, an attempt to merge into a block you may not own. The page refuses it by name. Supernetting is real (route aggregation does exactly this), but it happens between organizations that own the pieces, not inside one block.
What does the child step mean?
Neighbors in the carve are exactly one block apart: the step is 2 to the (32 minus child prefix) addresses. For /24 children the step is 256 — 10.0.0.0, 10.0.1.0, 10.0.2.0. The step is the number you type into a spreadsheet when you lay the plan out by hand.
Can I carve straight to /32s?
Yes, arithmetically — a /16 to /32 borrow yields 65,536 single-host routes, each with its own reserved pair made meaningless by size. Real designs stop at /30 or /31 for links; this page computes whatever depth you type and lets the per-child ledger show what the depth costs.
How does this differ from the subnet page?
The subnet page reads one address's neighborhood — where it sits, what its mask means, how many hosts its block can hold. This page plans from the block down: how many children a parent yields and where they land. Read versus plan — both need the other to close the loop.
Do the children overlap the parent's reserved pair?
The first child's network address is the parent's network address, and the last child's broadcast is the parent's broadcast — the parent's pair is not extra space; it is absorbed into the edges of the first and last children. Nothing is lost and nothing is double-counted.