There are couple of differences between IPv6 and IPv4 address allocation.
- The prefix length for an IPv6 subnet will always be /64; no more, no less. It allows you to place as many IPv6 devices as the underlying network medium allows. The 128 bit IPv6 address is automatically created from the /64 bit subnet extended with the 64 bit MAC-address of the NIC.
With IPv4, prefix length varies between subnets to subnets, and it caused painful costs when renumbering subnets (for example, imagine when you renumber an IPv4 subnet from /28 to /29 or vice versa).
- An ordinary leaf site will always get /48 of address space. This will be sufficient for most small to medium sized networks.
With IPv4, the allocation varies by the size of the site, and made it very painful when you migrated from one ISP to another, for example.