Addressing for IPv6 point to point links

The recommended best practice for addressing point to point links in IPv6 has been under discussion.

The original notion was simply to use EUI-64 and let the addresses autoconfigure. The problem with this approach is that the IP addresses of router interfaces often end up coded within parts of the router's configuration. Those hard-coded addresses then hurt when the interface becomes faulty, is replaced, and a different EUI-64 address is derived from the new interface's different MAC address. Hard-coding MAC addresses is a poor idea; protocols which have required that (eg, DECNet) were historically operationally painful.

A better notion is to use a /64 and configure …:1 at the more central interface and …:2 at the further interface. Not everyone is taken with the waste of addressing of this approach. IPv6 is big, but it won't be big for too long if addressing is squandered.

At the other extreme is a /127, using special features to nullify the Subnet Router Anycast Address (which no one much uses). The notion is problematic enough to warrant yet another Considered Harmful. Supporting a /127 is one more checkbox you need to test during equipment validation. And what if you have a nice bit of kit without the feature?

The suggestion which makes most sense to me is to use a /124. This has enough addresses to avoid any reuse of special addresses. It is not as wasteful as a /64. Best of all it is operationally sane, since four bits¹ make one textual hexadecimal character in the IPv6 address. So the more central router can be …:abc1/124 and the further router can be …:abc2/124. That's a lot easier to manage than dealing with non-character boundaries.

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¹ Four bits is commonly called a nybble, back-formed from eight bits making up a byte.