I think /64 would be the correct subnet size to ban in most cases. My understanding is that /64 has become the de-facto standard subnet size for IPv6 sites. Fo

I think /64 would be the correct subnet size to ban in most cases. My understanding is that /64 has become the de-facto standard subnet size for IPv6 sites. For example, I think that most home and business internet connections would be likely to receive a /64 allocation from their provider. Moreover, many times the hosts will be generating themselves a new random address (within the /64) from time to time. So blocking a single IPv6 address probably won't block the user for long. Conceptually, whenever a site (or hosted server, etc) would have been allocated a single address (/32) under IPv4, I think the analogous allocation for IPv6 is a /64, i.e.: 1.8 x 10^19 addresses. If you look at RFC5375, you can see why /64 would be common: - "An allocation of a prefix shorter then 64 bits to a node or interface is considered bad practice..." - "Using subnet prefixes shorter than /64 would rarely be useful..." - "Using subnet prefixes longer than /64 is not recommended for general use, and using them for links containing end hosts would be an especially bad idea..." - "Using a subnet prefix length other than a /64 will break many features of IPv6, including Neighbor Discovery, Secure Neighbor Discovery, privacy extensions, parts of Mobile IPv6, PIM-SM with Embedded-RP, and Site Multihoming by IPv6 Intermediation, among others."