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The Case for the /127 Subnet - Part I

June 30, 2014

In my previous post I wrote about all the positives we get from the almost incomprehensibly massive IPv6 address space, all there for our enjoyment if we will just break free of our long-ingrained IPv4 address conservation mentality.

The place where we most often see the adverse effects of IPv4 conservatism is in addressing point-to-point links. I wrote about how we can be led into illogical thinking because we fail to grasp the scale we are working with: We seem to have no problem on a LAN with 5000 addresses wasting the other 264-5000 addresses, but we just can’t bring ourselves to waste 264-2 addresses on a point-to-point link. On the scale of 18 million trillion addresses, the difference between 5000 and 2 is negligible.

A bit of illogic related to this issue of subnet address size is a question I hear frequently: “Why did they use up an entire 64 bits for the Interface-ID? Why not 32 bits, which still gives us more addresses on a subnet than we would ever use and would leave us with 96 bits for prefixes?”

My answer is: Why do you care?

That’s still just a misplaced worry about waste. We could just as easily ask why they didn’t make IPv6 addresses 64 bits with 32 bits for prefixes (location) and 32 bits for Interface-ID (identity). There would probably still be more than enough addresses for the foreseeable future. And you just know that if an IPv6 address were 64 bits, there would be people asking why they didn’t make the Interface-ID 16 bits (65,536 addresses should still be more than enough for any subnet!) so that we could have 48 bits for prefixes.

Do you see the kind of rut you can get stuck in by worrying about address waste? IPv6 addresses are 128 bits, evenly divided between prefix and Interface-ID, precisely because it gives us more addresses than we can ever conceivably use. The designers hit the problem of address depletion with a really, really big hammer. Unlike IPv4, there is no expectation that you will efficiently use all the interface addresses available on a /64 subnet.

So really. Stop worrying about it.

Read the next post in this series.

Note: This post originally appeared on Network World article titled “The Case for /127 Subnets.”

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