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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.

"You want me to allot an IP subnet with 18 million trillion addresses to a link that will only ever use two of them? Are you kidding me?" We know all the arguments for what we get in exchange for squander: Easier address management with one-size-fits-all subnets; simpler address interpretation; scaling; flexibility.

But still. Only using two addresses out of 18 million trillion? (Saying "million trillion" is a lot of fun if you imitate Carl Sagan’s voice.) Well, ask yourself when a /64 is acceptable.

Most people would say they can accept it on a regular LAN or VLAN segment. All righty then. To be fair, let’s take a really big LAN. Say, 5000 devices. Is a /64 acceptable there? Yes, you say? So we’re wasting (1.8 x 1019) – 5000 addresses instead of (1.8 x 1019) – 2 addresses. The difference between 5000 and 2 relative to 18 million trillion is miniscule. It diminishes to practically nothing. If it were any smaller it would be the amount I’m being paid to write this.

And yet a /64 on a LAN is acceptable and a /64 on a point-to-point link is not. IPv4 thinking can twist our reason. All of this does not mean there are not reasons to use a prefix other than /64 on point-to-point links – it only means address waste is not one of them. In fact, there are dueling RFCs on the topic.

RFC 3627 makes its case right in the title: "Use of /127 Prefix Length Between Routers Considered Harmful." The central argument in the document, however, is not as striking as the title suggests. Here it is:

  • When you use a /127 prefix on a point-to-point link, you have exactly two addresses available: PREFIX::0/127 and PREFIX::1/127. The problem the RFC cites is that the router being assigned PREFIX::1/127 might add the Subnet-Router Anycast address, which would be PREFIX::0/127. Then the router on the other end of the link, configured with PREFIX::0/127 will fail the Duplicate Address Detection test.
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