Hi Sarah,

Please have a look at ARIN's Number Resource Policy Manual (NRPM), Version 2010.2 - 7 June 2010, Section 6:
https://www.arin.net/policy/nrpm.html#six

Some samples from the document follow:

"RFC 2373, RFC 2373bis designate 2000::/3 to be global unicast address space that IANA may allocate to the RIRs. In accordance with RFC 2928, RFC 2373bis, IAB-Request, IANA has allocated initial ranges of global unicast IPv6 address space from the 2001::/16 address block to the existing RIRs. This document concerns the initial and subsequent allocations of the 2000::/3 unicast address space, for which RIRs formulate allocation and assignment policies."

"Organizations that meet the initial allocation criteria are eligible to receive a minimum allocation of /32.

Organizations may qualify for an initial allocation greater than /32 by submitting documentation that reasonably justifies the request. If so, the allocation size will be based on the number of existing users and the extent of the organization's infrastructure."

"End-users are assigned an end site assignment from their LIR or ISP. The exact size of the assignment is a local decision for the LIR or ISP to make, using a minimum value of a /64 (when only one subnet is anticipated for the end site) up to the normal maximum of /48, except in cases of extra large end sites where a larger assignment can be justified.

The following guidelines may be useful (but they are only guidelines):

    * /64 when it is known that one and only one subnet is needed
    * /56 for small sites, those expected to need only a few subnets over the next 5 years.
    * /48 for larger sites"

"Organizations that meet the direct assignment criteria are eligible to receive a direct assignment. The minimum size of the assignment is /48. Organizations requesting a larger assignment must provide documentation justifying the need for additional subnets."

Kind Regards,
Maria