IPv6 VLSM Planner¶

The VLSM IPv6 tab allocates IPv6 subnets of different sizes from a single parent block, mirroring the IPv4 VLSM Planner but with full 128-bit arithmetic and IPv6-specific idioms.

Unlike IPv4, every IPv6 address in an allocated block is usable — there is no broadcast or network address reservation. This means a /64 block has exactly 2^64 usable addresses.

How it works¶

Enter the parent network (e.g. 2001:db8::) and prefix length (e.g. /32).
Add rows — one per subnet — with a name and the number of hosts needed.
Click Calculate.

The planner sorts subnets largest-first, then greedily allocates the smallest power-of-2 block that satisfies each requirement from the remaining address space.

Host count input¶

The Hosts Needed field accepts:

Input	Meaning
`256`	A positive integer (decimal)
`2^64`	Two raised to the Nth power, where N is 0–128

Use 2^N for very large IPv6 sizings that exceed PHP's 64-bit integer range. For example, a /48 site allocation in a /32 ISP block holds 2^80 addresses — typing 2^80 is far easier than the equivalent decimal.

Result table columns¶

Column	Description
Name	Label you entered
Hosts Needed	Your requested host count (as entered)
Allocated Subnet	The IPv6 CIDR block assigned
Usable	Total addresses in the block (shown as `2^N` for very large blocks)

There is no separate "Waste" column for IPv6 — with 128-bit address space, the concept of "waste" is rarely meaningful, and block sizes are typically chosen along the natural /48 / /56 / /64 boundaries.

Allocation behaviour¶

Largest-first sort. Requirements with the most hosts are allocated before smaller ones, so block boundaries align cleanly without wasting address space.
Power-of-two block sizing. Each requirement gets the smallest /p block (where p is between the parent prefix and /128) such that 2^(128 - p) >= hosts.
Boundary alignment. Each block starts on its natural boundary (e.g. a /64 always starts on a 64-bit boundary).
Over-capacity error. If the total request exceeds the parent block, the planner reports the offending requirement.

Export¶

Button	Output
Export CSV	Comma-separated values file
Export JSON	JSON array of row objects
Export ASCII	Plaintext tree diagram copied to clipboard

XLSX export is intentionally omitted for IPv6 — the 2^N strings used for very large blocks do not round-trip cleanly through Excel cell formats.

Shareable URL¶

The planner produces a shareable URL that round-trips the parent network, prefix, and all requirements via GET parameters. See Shareable URLs & Embedding for details.

REST API¶

The same allocator powers the POST /api/v1/vlsm6 endpoint. See the REST API reference for the request/response shape.