Network Engineer
IPv6 Subnet Size Calculator
Use this ipv6 subnet size calculator entirely in your browser.
Addresses per /64: 18,446,744,073,709,551,616.
Borrowed subnet bits: 16.
Calculations happen locally in your browser.
How to use this calculator
- Enter the labelled values and review the live result.
- Review the live result and use Copy result when you need to share it.
How the calculation works
Borrowing n prefix bits creates 2^n child subnets; a /p subnet contains 2^(128-p) addresses.
Formula
Borrowing n prefix bits creates 2^n child subnets; a /p subnet contains 2^(128-p) addresses.
Step-by-step calculation
- Enter the labelled values and review the live result.
- Borrowing n prefix bits creates 2^n child subnets; a /p subnet contains 2^(128-p) addresses.
- Check the displayed result, then copy it if needed.
Worked example
Splitting a /48 into /64 networks creates 65,536 subnets.
Methodology and review
Who this is for
Network engineers planning IPv6 prefix subdivisions.
Inputs and units
- Allocated prefix
- Target child prefix
Method
The tool uses prefix-length arithmetic: borrowing n prefix bits creates 2^n child prefixes, while a /p prefix contains 2^(128-p) addresses.
Worked example
Splitting a /48 into /64 networks creates 65,536 child prefixes.
Assumptions
- Prefix lengths are integers from /0 through /128.
- Address counts describe mathematical capacity rather than a recommended allocation plan.
Common mistakes
- Counting IPv6 hosts like IPv4 usable hosts
- Reversing parent and child prefixes
Limitations
- The calculator does not discover, route, reserve, or validate an organisation's live IPv6 assignments.
- Operational subnet plans should follow the relevant provider and network policy.
How this tool was checked
Tests cover common /48, /56, and /64 splits plus invalid prefix ordering and boundary values.
Reference sources
Related tools
Calculation review: 21 August 2026. Published by Goldwave Dynamics Limited. This is an internal product review, not an independent certification.
Risk guidance
Frequently asked questions
Does this tool send my inputs anywhere?
No. The interactive calculation runs in your browser and this site does not submit tool inputs to a backend.
How should I use the result?
Check that the entered units and values match your situation, especially around dates, breaks, and rounding.
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