Dual Stack Planner
Compare IPv4 and IPv6 capacity for the same network and size the overhead of running both stacks.
About Dual Stack Planner
The two stacks are sized independently: IPv4 is constrained by scarcity and needs segment counts, while IPv6 is constrained only by the prefix you were allocated. Running both roughly doubles neighbour tables, routes and monitoring series. Dual stack is the default migration path, and the hidden cost is not addresses but the doubled state on firewalls, load balancers and monitoring systems.
Formula
Dual Stack
IPv4 segments = ceil(hosts ÷ (2^(32 − v4 prefix) − 2)); IPv6 /64 subnets = 2^(64 − site prefix).
- perSegment
- Usable hosts per IPv4 segment
- stateEntries
- Extra neighbour, route and session entries added by the second stack
Worked example
With the default values below, Dual Stack Planner returns IPv4 Segments Required of 16. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Hosts to addressaccepts 1–10,000,000 hosts
- 4,000 hosts
- IPv4 prefix per segmentaccepts 8–30 bits
- 24 bits
- IPv6 site prefixaccepts 32–64 bits
- 48 bits
- Extra state entries per hostaccepts 0–50 entries
- 3 entries
Results
- IPv4 Segments Required
- 16
- IPv6 /64 Subnets Available
- 65,536
- Additional Forwarding/State Entries
- 12,000
- IPv4 Headroom
- 1.6%
Step by step
- 1Values used
- 2Dual Stack
- 3IPv4 Segments Required= 16
- 4IPv6 /64 Subnets Available= 65,536
- 5Additional Forwarding/State Entries= 12,000
- 6IPv4 Headroom= 1.6
Assumptions and limits
- The result is derived only from the 4 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 1 decimal place, 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
How is Dual Stack calculated?
IPv4 segments = ceil(hosts ÷ (2^(32 − v4 prefix) − 2)); IPv6 /64 subnets = 2^(64 − site prefix). The two stacks are sized independently: IPv4 is constrained by scarcity and needs segment counts, while IPv6 is constrained only by the prefix you were allocated. Running both roughly doubles neighbour tables, routes and monitoring series.
Why does Dual Stack matter?
Dual stack is the default migration path, and the hidden cost is not addresses but the doubled state on firewalls, load balancers and monitoring systems.
What values do I need to enter?
This calculator takes 4 inputs: Hosts to address, IPv4 prefix per segment, IPv6 site prefix, Extra state entries per host. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.