• 10/04/2011
    8:28 AM
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The Fear And Loathing Of /64s On Point-To-Point Links

I discussed in a previous article the necessity of abandoning IPv4 thinking when creating IPv6 address designs, and how our deeply ingrained need to conserve addresses can muddle our thinking. Nowhere does this conservative aversion to address waste snarl at us as menacingly as when we consider – completely compliant with the recommendations of ARIN and other RIRs – assigning /64 subnets to point-to-point links.
  • The reason this issue is not really much of a concern is that the Subnet-Router Anycast address should not be needed or used on a point-to-point link. In fact, the RFC itself states that this problem has not been observed in general, probably because the Subnet-Router Anycast address is not widely used.

    At the same time, the mandated use of /64 subnets to support such functions as Stateless Address Autoconfiguration, PIM-SM with embedded RP addresses, and various Neighbor Discovery functions are not relevant to point-to-point links where these functions are not used. That’s not to say that there will never be a case where some underlying support function needed on a point-to-point link will require a /64 to work; the base IPv6 specification does expect to see a 64-bit Interface ID. But this is speculative, and not really a compelling reason to make an address design choice.

    The real argument for using /64 on point-to-point links remains what I have already stated: The simplicity, consistency, and flexibility of using a single subnet size throughout your network.

    This usage is also supported in the standards. Section 3 of RFC 5375, “IPv6 Unicast Address Assignment Considerations,” plainly states: “Using /64 subnets is strongly recommended, also for links connecting only routers. A deployment compliant with the current IPv6 specifications cannot use other prefix lengths.” There you have it in black and white, ladies and gentlemen.

    There is also no need to worry that the addressing bodies are going to penalize you for being wasteful. Here’s what ARIN says in its IPv6 Address Plan General Guidelines: “No subnets will use prefixes longer than /64.” And later on the same page: “The IETF expects that you will assign a /64 for point-to-point links.”

    So is there a case to be made for using /127 subnets? Well, yes.

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