IPv6 Subnet Planner

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.