IPv6 Subnet Planner
Divide a delegated prefix into subnets. IPv6 planning is about counting subnets, not counting hosts — this does not pretend otherwise.
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Segments
Why IPv6 planning does not count hosts
An IPv4 plan starts with "how many devices?" because addresses are scarce. An IPv6 /64 contains 18.4 quintillion addresses, so the question is meaningless — a /64 holding four devices and a /64 holding four thousand are the same allocation.
Why /64 and not something smaller? Stateless address autoconfiguration, the mechanism by which hosts assign themselves an address without a DHCP server, expects 64 bits of interface identifier. Subnet smaller than a /64 and it stops working. So a /64 is the standard subnet size regardless of how few hosts sit in it, and using a /120 "to save space" saves nothing and breaks things.
What you are actually counting is subnets. A /48 holds 65,536 /64s. A /56 — the common residential delegation — holds 256. That is the number worth planning against.
Point-to-point links are the recognised exception. RFC 6164 recommends /127 for router-to-router links, to avoid a neighbour-discovery exhaustion issue that /64 links are exposed to. Some operators still assign a /64 and configure a /127 from it.
Leave gaps. With this much space there is no reason to allocate contiguously; grouping by site or function with room between them makes summarisation possible later.
Allocate on nibble boundaries
An IPv6 address is written in hexadecimal, and each hex digit carries exactly four bits — a nibble. Prefix lengths that are multiples of four therefore land on a digit boundary, and prefix lengths that are not, land in the middle of one.
That is the whole argument for planning at /48, /52, /56, /60 and /64 rather than at /61 or /58. A /61 is perfectly legal and the arithmetic works, but its subnets differ in the middle of a hex digit, so you cannot look at two addresses and see which allocation each belongs to without doing binary in your head. On a nibble boundary the allocation is legible: the digit itself is the subnet number. Every later act of reading a log, writing a filter or explaining the plan to somebody is easier for it, and the cost is address space you have in overwhelming abundance.
The delegation sizes you will meet
A /64 is one segment — the standard size for any single link, regardless of how many hosts sit on it. A /56 gives 256 of those, and is a common delegation to a home or small site. A /48 gives 65,536 and is the usual allocation to a site or an organisation, which is also the size registries expect an end site to be able to receive.
The instinct carried over from IPv4 is to hand out the smallest thing that fits, and it is the wrong one here. Requesting a /56 when a /48 was available, to be economical, produces a plan that runs out of structure long before it runs out of addresses — and renumbering later costs far more than the space ever would.
Related tools
IPv6 Visualizer explains a single address. Subnet Calculator handles both families.