Split an IPv6 allocation into child prefixes
This calculator takes a parent IPv6 allocation such as 2001:db8:1234::/48 and divides it logically into more-specific child prefixes such as /56 or /64. It calculates how many child networks are available, shows the first and last child, and provides a small preview starting from any index you choose.
The tool is designed for planning rather than dumping enormous lists. A /48 contains 65,536 different /64 networks, so indexed previews are much more useful than attempting to render every child at once. The IPv6 guide provides the conceptual background.
How to use the IPv6 subnet calculator
Enter the parent allocation in CIDR notation, choose a child prefix length that is equal to or more specific than the parent, and set an optional starting index. Press Calculate subnets to normalize the allocation and generate a preview.
The result shows the total number of child prefixes, the first child, the last child and the requested slice of the sequence. The start index is a non-negative integer, so you can jump to a distant part of a very large allocation without enumerating everything before it.
Example: divide a /48 into /64 networks
Moving from /48 to /64 borrows 16 additional bits for subnet identification. That produces 2^16 = 65,536 child /64 prefixes. The first child begins at the normalized parent boundary, while the final child has every borrowed subnet bit set to one.
If you only need the network boundary for a single address and prefix, use the IPv6 prefix calculator instead.
Parent and child prefix rules
The child prefix cannot be shorter than the parent because that would describe a larger network rather than a subdivision. A /56 can be split into /60 or /64, but it cannot be “split” into /48. Equal prefix lengths are valid and simply produce one child: the parent itself.
The calculator normalizes the address to the real parent boundary before deriving child networks, preventing host bits in the input from shifting the plan.
Why IPv6 subnetting is different from IPv4 host math
IPv4 subnet exercises often focus on usable-host counts, network addresses and broadcast addresses. IPv6 planning is normally about prefix hierarchy and address allocation. There is no IPv4-style broadcast address to subtract, and a /64 contains an enormous interface identifier space.
The useful question is often how many child prefixes fit inside a delegated block, not how many individual addresses can be assigned. This is why the tool emphasizes prefix count and hierarchy.
Planning with /48, /56 and /64
Organizations may receive a larger prefix and then allocate smaller blocks to sites, buildings, services or VLANs. A consistent hierarchy makes route summarization, documentation and future growth easier. The exact plan depends on the delegated space and operational requirements, not on a universal one-size-fits-all template.
Use the IPv6 notation tool when you need to normalize an individual child prefix for documentation.
Frequently asked questions
Why does the tool show only a preview?
IPv6 allocations can contain huge numbers of child prefixes. Indexed previews avoid generating impractically large lists.
Can a /48 be split into /56 and /64?
Yes. Both are more-specific child prefixes of a /48.
Does IPv6 subnetting reserve network and broadcast addresses?
IPv6 does not use an IPv4-style broadcast address, so the classic “subtract two hosts” rule does not apply.
Can I start the preview at a large subnet index?
Yes. The calculator uses arbitrary-size integer indexing so you can inspect distant child prefixes without enumerating all earlier ones.
Next step
Use the preview to validate your hierarchy, then document the allocation plan and connect each child prefix to the appropriate routing, VLAN or service design.