Not long ago, ARIN changed the IPv6 policy so that residential subscribers could be issued with a /56 instead of the normal /48 assignment. This was done so that ISPs with large numbers of subscriber sites would not exhaust their /32 (or larger) allocations too soon. Since these ISPs are allowed to assign a /56 to residential subscriber sites, their initial IPv6 allocation will last a lot longer and they won't have to apply for an additional allocation while everyone is getting up to speed with an IPv6 Internet. Now, however, the IETF is discussing a form of NAT for IPv6 called NAT66. See this draft for details <http://www.ietf.org/internet-drafts/draft-mrw-behave-nat66-01.txt> Part of this new NAT is that they are checksum neutral. They do this by modifying bits in the address that are not needed. Specifically, they assume that the end site has a /48 allocation, and that the next 16 bits up to the /64 boundary, are non-essential information outside the end-site boundary. These bits are then twiddled to preserve the IPv6 header checksum. Of course, these are the same bits that an ISP relies on for reducing the assignment size to /56. I see a potential conflict here. If we assume that NAT66 will be widely used by consumers, then it follows that consumer end-sites will need a /48 assignment in order for IPv6 to work. But some ISPs want to reduce the end site assignment to /56 meaning that NAT66 won't work for those consumers. Of course, it's not all set in stone yet which is why I am posting this to NANOG. If ISP's who intend to use /56 allocations could join in the discussions, then perhaps we could develop some form of NAT66 that works with /56 prefix lengths. Personally, I would be happy to just see every site consistently use a /48 assignment. Corporate campus or one-room studio apartment; it's all the same to me. --Michael Dillon