Creating our first IPv6 address plan presents another minor catch-22 scenario: it’s a little more complicated to construct an address plan without knowing up front how much IPv6 address space we’ll need. On the other hand, it’s also difficult to know how much IPv6 address space to request until we’ve worked through enough of our plan to be confident that we’re asking for the right amount.
So we’ll actually have to do a bit of address planning design before we request an IPv6 allocation. While we’ve established that the enormous size of the typical allocation makes sufficient host addressing concerns generally irrelevant, we still need enough groups of subnets to make sure our address plan provides operational efficiency and manageable growth.
Keep in mind that since we’re constructing an address plan geared toward operational efficiency, one where conservation of address space is in nearly all cases a negligible concern, it should be possible to use the ideas and methods in this book to develop a plan based on the assumption that we’ll be granted the address space we need from the allocating agency. While a rough estimate of the right amount of address space may be useful to help us get started with developing our plan, we should be perfectly comfortable with coming to realize as we work through the planning process that we need more (or less) address space than we initially assumed.
Your plan should drive your ultimate allocation size. ...