Mark,
Only short sighted ISP's hand out /56's to residential customers.
I am curious as to why you say it is short sighted? what is the technical or otherwise any other reasoning for such statement ?
256 is *not* a big number of subnets. By restricting the number of subnets residences get you restrict what developers will design for. Subnets don't need to be scares resource. ISP's that default to /56 are making them a scares resource.
The excerpt Royce quoted from RFC6177 (requoted below) seems to back away from /48s by default to all resi users and land in a somewhat vague "more than a /64 please, but we're not specifically recommending /48s across the board for residential" before specifically mentioning /56 assignments. The general push in the community is towards /48 across the board. Any comments on why the RFC backs away from that? Is this just throwing a bone to the masses complaining about "waste"? btw: hat tip to Peter Rocca for a kind of scale we're talking about for allocatable space. -- Hugo
Quoting RFC6177 (successor to RFC3177):
While the /48 recommendation does simplify address space management for end sites, it has also been widely criticized as being wasteful. For example, a large business (which may have thousands of employees) would, by default, receive the same amount of address space as a home user, who today typically has a single (or small number of) LAN and a small number of devices (dozens or less). While it seems likely that the size of a typical home network will grow over the next few decades, it is hard to argue that home sites will make use of 65K subnets within the foreseeable future. At the same time, it might be tempting to give home sites a single /64, since that is already significantly more address space compared with today's IPv4 practice. However, this precludes the expectation that even home sites will grow to support multiple subnets going forward. Hence, it is strongly intended that even home sites be given multiple subnets worth of space, by default. Hence, this document still recommends giving home sites significantly more than a single /64, but does not recommend that every home site be given a /48 either.
A change in policy (such as above) would have a significant impact on address consumption projections and the expected longevity for IPv6. For example, changing the default assignment from a /48 to /56 (for the vast majority of end sites, e.g., home sites) would result in a savings of up to 8 bits, reducing the "total projected address consumption" by (up to) 8 bits or two orders of magnitude. (The exact amount of savings depends on the relative number of home users compared with the number of larger sites.)
The above-mentioned goals of RFC 3177 can easily be met by giving home users a default assignment of less than /48, such as a /56.