This is a good explanation. To elaborate on the /64 network assumption, it has to do with stateless autoconfiguration, which is the way of configuring IPv6 add

This is a good explanation. To elaborate on the /64 network assumption, it has to do with stateless autoconfiguration, which is the way of configuring IPv6 addresses. An IPv6 address is comprised of a 64 bit prefix followed by a 64 bit "interface ID." The interface ID is inherent to an interface, and is derived from the MAC address, since it is guaranteed to be globally unique. When you bring up an interface, the OS listens for router advertisements which contain the 64 bit prefix. It takes this prefix and appends the interface ID to generate a full IPv6 address which is unique. This is really cool because it means your computer will always have the same lower 64 bits no matter where you take it, and it can auto-discover the prefix without the complexities of DHCP. (Note that there were some concerns about tracking computers using the lower 64 bits, which led to the optional privacy extensions, under which the interface ID is randomly generated.) However, the standard requires that the interface ID be 64 bits, so if you only have a /64 from your ISP you are, practically speaking, limited to 1 subnet. For this reason, I like to measure prefix lengths in terms of subnets rather than IPv6 addresses.