Because that's the minimum block size (/64) for several IPv6 features - mostly important the stateless address configuration - to work. I'd thought of explaini

Because that's the minimum block size (/64) for several IPv6 features - mostly important the stateless address configuration - to work. I'd thought of explaining with my own words, but found very good and detailed explanation here: http://serverfault.com/questions/182881/why-add-an-ipv6-address-as-64/218432#218432 http://serverfault.com/questions/182881/why-add-an-ipv6-addr..., so I'd just quote. --- begin quote -- The IPv6 addressing architecture (RFC4291) requires all unicast addresses always have a prefix length of 64 bits. Using something other than a /64 will break a number of IPv6 features such as neighbor discovery, secure neighbor discovery (SEND), privacy extensions, mobile IPv6, embedded-RP (multicast), etc. The current IPv6 address assignment guidelines call for allocating a /64 only when it is known that one and only one subnet is needed, otherwise a /56 should be allocated to small sites that are "expected to need only a few subnets over the next 5 years", or a /48 for larger sites. (ARIN policy 6.5.4.1) --- end quote --- This is why all those just-bit-counting "number of atoms in universe" and "79 octillion times" comparisons are wrong. Actual comparison should compare single IPv4 address with something ranging from /64 to /48 IPv6 allocation - /64 is a minimum per L2 broadcast segment - so it's a minimum per-customer allocation (unless ISP is really greedy). Also, I don't know of reasons, but RIPE NCC themselves uses /32 IPv6 block with about /21 IPv4 one comparison to calculate LIR billing score - http://ripe.net/membership/billing/calculation.html http://ripe.net/membership/billing/calculation.html