These kinds of numbers mean that you can trade traditional address conservation principles for all kinds of other benefits: consistent, one-size-fits-all subnets; addresses that are easier to interpret, troubleshoot and maintain at the hex level without needing to look at the binary representation; and an address design with the flexibility and scalability to meet future network requirements, even if you're not sure what those requirements might be.
Yet, when I've done IPv6 address designs for clients, convincing at least some of the architecture team to forsake long-ingrained IPv4 thinking can be a tough sell. "Why do I need 1.8x10^19 x 1,019 addresses on a subnet? Any sanely built subnet will have only a miniscule fraction of that number of devices." The objections can get particularly strenuous when we start talking about using a /64 on point-to-point links. "Why am I assigning 180 million trillion addresses to a link that will only ever use just two of them?" you might ask.
These objections miss the point. It's not about how many addresses you are wasting; it's about what you get in return for being wasteful. After all, what else are you going to do with all those addresses? If you cannot address your entire network with 64-bit subnets and still have plenty to spare, you haven't been given a prefix appropriate to the size of your network.
There are a couple of reasons other than address conservation why you might want to consider a different subnet size on your point-to-point links, and I'll discuss them in a later post.
I'll admit that leaving all those nice IPv6 addresses to never ever be used sometimes bothers me, too. I've been working with IPv4 for a very long time and am far from immune to having traditional sensibilities offended. That's why it's important to not think generically about IP address design. There's IPv4 design, and there's IPv6 design. Two different sets of practices.Jeff Doyle is president of Jeff Doyle and Associates, Inc., a consultancy specializing in large scale service provider network design, IPv6, and network best practices. View Full Bio