As far as I've understood, they wanted a little of everything. IPv6 addresses should be more static short to medium term, because you don't have to reassign. Y

As far as I've understood, they wanted a little of everything. IPv6 addresses should be more static short to medium term, because you don't have to reassign. Your local pool is basically infinite, so e.g. a client logging off the WiFi doesn't mean you immediately have to recycle the address. The local part (the latter 64 bits) is enough for everything and everyone. Now, the first 64 bits should be used for routing, which should preferably be more hierarchical than with IPv4. The idea was that a customer/company/small ISP would get a prefix assigned that was below the hierarchy of the upstream ISP (with IPv4, that hierarchy isn't possible because there aren't enough address bits, and lots of lower-tier customers got their own subnets). So a large ISP might be assigned 2345::/16, assign 2345:6789::/32 to a smaller ISP who might assign 2345:6789:abcd::/48 to a customer (who then will do more subnetting with that). That keeps the global and local routing table small, since everyone just needs to look at the address prefix to know where to route to. If the customer now moves to another ISP, he is assigned a new prefix (e.g. 4711:f0f0:666::/48) by the new ISP and needs to renumber, meaning that all the customers addresses keep their suffix but change their prefix. Protocols like SLAAC where designed as stateless to support (among other things) quick and easy renumbering. Just announce the new prefix, wait a TTL and you are done. Clients automatically switch over to the new addresses, in an ideal world this is mostly transparent to the end user. In the new millenium, especially after Snowden, the argument of privacy got stronger, meaning that randomised local /64 suffixes are now en vogue, as opposed to the earlier MAC-based static suffixes. Therefore everything is more dynamic and also more static ;)