Monday, September 26, 2016

Extending a /64 Prefix

by Craig Miller

Extending a /64 Prefix
It would appear that ISPs are still learning how to deploy IPv6, and the old "conserve address space" thinking continues. Amazingly there are many ISPs still using a single /64 for their customers, with no Prefix Delegation (PD).

Some math, how far will a /32 go?

ISPs with only a /32, and not giving their customers more than a /64, have failed to understand just how big a /32 is. With a /32, there are only 4 billion subnets to give out to their customers. If the ISP gives out an additional /64 (using DHPCv6 PD) to each customer, that is still 2 billion customers, each with a /64 in their home network.

But what if the ISP was a little more generous, and gave out a /56, allowing customers to subnet within their home (255 subnets)? That /32 would be exhausted only after 16 million customers (or about the total number of internet customers of AT&T in 2016 Q2. That is just one /32, if an ISP runs out, they can apply for another. As I have stated before, we need to shed the "conserve address space" thinking of IPv4 when doing network planning in IPv6.

Wireless 3GPP and an IPv6 Hotspot

But what if your ISP only provides a /64 for the end device with NO additional subnets. Telstra does this with their wireless network. Phones only need to attach to the IPv6 wireless network, right?

What if the customer wanted to run a hotspot? Since there is no additional /64 for the "LAN" network what is one to do? Fortunately there are smart minds suggesting solutions. OpenWRT has a "relay-mode" which relays RAs and NDP traffic from upstream, thus extending the WAN-side /64 prefix into the LAN-side. There is also the v6brouter project which uses bridging for IPv6 while maintaining IPv4 NAT.

Standards to the Rescue

And there is RFC 7278 Extending an IPv6 /64 prefix from a 3GPP mobile interface to a LAN link. The RFC describes processing RA and NDP packets which sounds very similar in concept to the OpenWRT/odhcpd implementation.

The good news is that ISPs are learning more about IPv6 deployment. If you are one of the unlucky ones with only a /64 or less (no PD), comment to your ISP, and let them know there is plenty of room. And if you want to set up that IPv6 enabled hotspot on your phone, go ahead, and thank the authors of RFC 7278.

* extension cord, creative commons

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