IPv6 Subnet Calculator
Calculate exact IPv6 ranges and preview indexed child subnets
Exact local calculation. Preview 1–256 rows; the new prefix must be at least the parent prefix. Indices start at 0 and require decimal integers. IPv6 has no broadcast address; zone suffixes are not accepted.
Calculate to see the normalized CIDR, range, and subnet capacity.
About this tool
IPv6 Subnet Calculator normalizes a parent CIDR and previews child networks without enumerating their addresses. The original address, normalized parent, inclusive endpoints, exact capacity, child count and start index are shown together. A /48 split into /64 networks contains 65,536 children; index 16 selects the seventeenth. Prefixes and indices use complete decimal integers, and preview count must be 1–256. Indices above 2^53 remain exact through BigInt. Editing any field clears the completed preview, so copied CIDRs and the remaining count describe the same calculation. The last address is a range endpoint, not a broadcast or proof that every address is assignable.
Production Snippets
A preview can end before the requested row count
text
Parent input: 2001:db8::53/120
Child prefix: 124
Start index: 14
Preview count: 8
#14 2001:db8::e0/124 [2001:db8::e0 … 2001:db8::ef]
#15 2001:db8::f0/124 [2001:db8::f0 … 2001:db8::ff]
There are only 16 children (indices 0–15).
After these two rows, zero children remain.
The original ::53 host bits do not change the /120 network boundary.Frequently Asked Questions
How are the parent and child prefixes related?
Both are within /0–/128. The child prefix must equal or exceed the parent prefix. A /120 split into /124 yields 16 children, numbered 0–15.
Can I use an index larger than JavaScript’s safe integer range?
Yes. The index is parsed directly as a decimal BigInt. For ::/0 split into /128, the last valid index is 340282366920938463463374607431768211455.
What does the remaining count mean?
It counts children after the last displayed row. Children before the selected start index are not included in that number. At the final child it is zero.
Are blank or fractional preview settings rounded?
No. Preview count must be an integer from 1 to 256. Empty values, fractions, signs and leading zeros are rejected. New prefix and index also require complete decimal integers.
Do host bits in the input change the parent network?
They are normalized away for network calculations while the original address remains visible. For example, 2001:db8::53/120 has parent 2001:db8::/120. An empty slash is invalid; /0 is valid.
Are IPv6 zones or network reachability checked?
No. These network calculations reject zone suffixes such as %eth0 and accept paired brackets around IPv6 addresses. Processing is local; address assignment, routing and reachability are outside the calculation.
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