The Fear And Loathing Of /64s On Point-To-Point Links
Posted by
Jeff Doyle
October 04, 2011
Some networkers are designating /126 prefixes for subnets rather than /127, on the misguided assumption that they should use an IPv6 equivalent of an IPv4 /30: four addresses per point-to-point subnet, two for the interfaces, one as the “subnet address” (host bits all zero) and one as the broadcast address (host bits all one).
One more time: IPv4 thinking. Not only that, it’s old even for IPv4 thinking. On a point-to-point link neither the subnet address nor the broadcast address is used for anything. And in IPv6, there is not a broadcast address at all, so the all-ones host address is functionally meaningless. It’s just another address.
If you insist on conserving IPv6 addresses on router to router links use /127. A /126 wastes two whole addresses, and we can’t have that for heaven’s sake.
If, despite all my ranting, you still just cannot decide whether a /64 or a /127 is best to use, you might consider a compromise: Reserve a /64 for each point-to-point link, but then configure a /127 out of the /64. If future best practice falls on the side of /127, you’re all set and can use the rest of the /64s elsewhere. If it remains on the side of /64s, you can do a simple prefix length change on your link addresses to bring your network into compliance. Whether you use 64-bit subnets or 127-bit subnets on your point-to-point links, be sure you are making your decision based on sound engineering reasons and not on outdated IPv4 design principles.
And remember that ARIN and the other RIRs support using /64s on all subnets. If you find that your IPv6 allocation does not support enough subnets for your network, you do not need to begin subnetting down into the Interface-ID. You need to ask your RIR for a larger allocation. I made that statement in a presentation a few weeks ago, and someone from ARIN stood up and affirmed it. They want you to use /64s, and they will allocate to be sure you can.
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