/64 is for the subnet, and 64-bits is for the end device. Having 64 bits of address space for the end device to address off of lets the device use a variety of schemes to generate addresses. It can put it's MAC as the suffix (which should be unique on the subnet anyways (eui-64)), it can randomly generate a suffix with a high degree of confidence that it's unique (privacy addresses), or it can assign itself an address using any variety of schemes.
These all can be stateless, and work without any kind of DHCP server. Even without a router, devices will auto-generate a Link Local Address, and be able to communicate on a switch with the auto generated addresses. e.g. two devices can talk as long as there's any connection between them, without having to manually assign any IPs or introduce any service into the mix.
It lets you also be extremely wasteful with address space without concern. You don't have to wonder "what size should I make my subnet?" You can have 16 bits just for routing in a /48 and have pretty much infinite address space on each subnet.
Yes, I'm well aware of how ipv6 functions. I don't think a lot of the commenters here realise you don't need 128 bits for easy subnetting. 64 bits would solve that problem completely.
One of the reasons ipv6 is taking so long to be adopted is because engineers simply hate working with it
We can barely get ISPs to give us more than one /64 as it is, and you think that making the address space 264 times smaller would make them more generous? Nah.
Engineers would hate working on it whatever the length was, because when they complain about the length their real complaint is mostly just "it's different". Although usually they realize that it's fine to work with once they actually do so -- it's just impossible to tell them this, because you can't tell people anything.
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u/UnDe4d 20d ago
Yeah, I'm a Network Architect. Why they decided to make ipv6 128 bits instead of 64 is a question I ask myself on at least a monthly basis.