I Got 1.2E24 Problems But a Site Ain't One

Few things seem to cause as much confusion and anxiety among network architects newly tasked with IPv6 address planning as the current recommended practice of allocating a /48 per site.

After all, a /48 is 1.2E24 addresses. Or, to put it another way, 65536 /64 subnets (each containing 1.8E19 addresses). That works out to 281 trillion Internets for a single site!

Yet a site is necessarily a logical construct, an abstraction that could accommodate locations ranging in size from a home network, to a multi-building campus network or a data center teeming with racks and servers. (And no one, least of all one of the oldest and most IPv6 connected service providers, would allocate a /48 to a home network!)

Confronted with this ambiguity, the tendency among those tasked with IPv6 address planning, is to revert back to "IPv4 thinking", an approach that privileges address conservation above all other concerns.

Yet this approach fails to recognize that the overwhelming abundance of a per-site /48 allocation in particular (and IPv6 in general) leads to opportunities to create a site addressing plan that maximizes simplicity and operational efficiency.

For instance, as explored in my recent blog post, the IPv6 subnets available in a /48 can be grouped hierarchically to allow for improved prefix aggregation as well as streamlined security policy design and compliance. Further, geographical and/or functional significance can be encoded into these subnet groupings, leading to improvements in operational efficiency. Such opportunities generally did not exist, or could not scale, in IPv4 due to address scarcity.

And recall that because the IPv6 address is represented hexadecimally, the 16 bits available between a single site /48 allocation and the typically irreducible interface allocation of a /64 are most legible when grouped on the nibble (or 4 bit) boundary. In my next post, I'll illustrate how this nibble boundary simplifies grouping the subnets available in a /48.

But in the meantime, as you move forward with your IPv6 address plan remember to resist the urge to start carving up a /48 allocation between multiple sites because you're afraid of running out of addresses. All indicators suggest that you'll be able to obtain all the /48s you need (and then some) from the allocating organization without having to compromise the efficiency of your per-site plan. 

As always, looking forward to hearing about your own experiences with IPv6 address planning in the comments!

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