Dual Stack Planner
Compare IPv4 and IPv6 capacity for the same network and size the overhead of running both stacks.
Inputs
IPv4 Segments Required
16
IPv6 /64 Subnets Available
65,536
Additional Forwarding/State Entries
12,000
IPv4 Headroom
1.6%
Step by step
Values used
Hosts to address = 4,000 hosts; IPv4 prefix per segment = 24 bits; IPv6 site prefix = 48 bits; Extra state entries per host = 3 entries
Dual Stack
IPv4 segments = ceil(hosts ÷ (2^(32 − v4 prefix) − 2)); IPv6 /64 subnets = 2^(64 − site prefix).
IPv4 Segments Required
= 16
IPv6 /64 Subnets Available
= 65,536
Additional Forwarding/State Entries
= 12,000
IPv4 Headroom
= 1.6
How it works
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
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.