How to plan IPv6 subnets

Stop counting hosts. In IPv6 the only number that constrains you is how many subnets your delegation contains.

Every habit from IPv4 addressing is about scarcity: size the subnet to the host count, avoid waste, reclaim unused space. None of it applies. A single /64 holds 18.4 quintillion addresses, so a segment with four devices and one with four thousand get the same allocation.

Count subnets instead

• A /48 contains 65,536 /64 subnets. The traditional site allocation.
• A /56 contains 256. The common residential or small-site delegation.
• A /64 is one subnet.

If your provider hands you a /56, you have 256 subnets. That, not any host count, is the number your plan has to fit inside.

Why /64 and not smaller

Stateless address autoconfiguration — hosts assigning themselves an address without a DHCP server — expects a 64-bit interface identifier. Subnet smaller than a /64 and it stops working.

So a /64 for twelve printers is not waste, it is the standard subnet size. Using a /120 to "save space" saves nothing you needed and breaks something you wanted.

The exception: point-to-point links

RFC 6164 recommends a /127 for router-to-router links. A /64 on such a link is exposed to a neighbour-discovery exhaustion problem, where an attacker scanning the enormous address space fills the router's neighbour cache. Many operators reserve a /64 for the link and configure a /127 out of it, which keeps the addressing plan tidy and the link safe.

Leave gaps

With 65,536 subnets there is no reason to allocate contiguously. Group by site or by function with room between the groups, so a route can summarise a whole site later. Packing allocations end-to-end to be tidy throws away the one thing the address space is generous with.

Frequently asked questions

How many devices fit in a /64?

About 18.4 quintillion. The number is not a useful planning input, which is why IPv6 planning counts subnets instead.

Is a /64 for a few devices wasteful?

No. It is the standard subnet size, and autoconfiguration depends on it. There are more than enough /64s in any normal delegation.

What should I ask my ISP for?

A /56 at minimum for a home or small site, and a /48 for anything larger. A single /64 leaves no room to subnet at all.

Should point-to-point links use a /127?

RFC 6164 recommends it, to avoid neighbour-cache exhaustion. Reserving a /64 and configuring a /127 from it is common practice.