Comcast is asking for volunteers to participate in its upcoming IPv6 trials. The cable ISP has been participating in IPv6 circles for a long time and with its huge subscriber base, it is experiencing the IPv4 address scarcity first-hand. So far, it has been able to get addresses for its customers—but not for those customers' cable modems and set-top-boxes. These also need addresses to function or to be managed. No problem, right? Just use private IPv4 addresses, such as the 10 network, which holds 16.8 million addresses. But with 25 million TV, 15 million ISP, and 6 million Comcast Digital Voice subscribers, 16.8 million private addresses isn't enough for a regular management system in which a management station can directly connect to each managed device. So Comcast needs IPv6 just to run its internal network effectively now.

We're also running out of IPv4 addresses, so at some point in the future, Comcast will be unable to obtain additional addresses to connect new customers. So Comcast also needs to provide IPv6 service to its customers at some point and is looking for willing subjects to give it a try.

Comcast plans four trials. The first one will use a transition technique that is still under development, called 6RD. 6RD is similar to the 6to4 automatic tunneling mechanism that is available in Windows (it's enabled automatically when the system has a public IPv4 address under Windows 7 and Vista). The difference is that 6RD only tunnels IPv6 packets across an IPv4-only part of the service provider's network, while 6to4 can tunnel across large parts of the Internet, possibly incurring slowdowns.

The second trial will be with native IPv6. Here, IPv6 packets are transmitted across the infrastructure without encapsulating them in IPv4 packets. IPv4 remains available, creating a "dual stack" deployment. "Native, dual-stack is central to our IPv6 strategy and we expect that the native dual-stack solution will be a significant part of the IPv6 transition, enabling IPv6 technology to evolve globally while still being able to provide seamless services to the traditional IPv4 Internet," says Comcast.

The third trial will basically be the opposite of the first: rather than encapsulate IPv6 packets in IPv4 packets in order to traverse IPv4-only network sections, IPv4 is now encapsulated in IPv6 to get across IPv6-only parts of the service provider network. The technique for this will be "Dual Stack Lite," a protocol that is being developed in the IETF Softwires working group, which is co-chaired by Comcast's Alain Durand. In Dual Stack Lite, the IPv4 hosts use private IPv4 addresses. A home router encapsulates those in IPv6 packets, and a "carrier grade NAT" both decapsulates the packets and performs Network Address Translation so that a lot of DS-Lite clients can share a single IPv4 address.

The fourth trial will evaluate how to provide IPv6 to business class customers. The first two trials are scheduled for the second quarter of 2010, the other two for the third quarter. At this time the details on what participating in each trial will encompass are unclear, and signing up doesn't necessarily guarantee participation.