Two explanations come to mind: 1. easier routing tables if you can add meanings to specific bit ranges of your ipv6 address. In the tightly assigned ipv4 netwo

Two explanations come to mind: 1. easier routing tables if you can add meanings to specific bit ranges of your ipv6 address. In the tightly assigned ipv4 networks we have arrived at this is a bit annoying. 2. If the ipv6 conventions were that you set, say the highest 5 hex groups to 0, and use the lowest 3 hex groups for addresses, it would still be 65536 times as large as the ipv4 space and would suit most needs for the mid term future. You could even write ipv6 addresses nicely using e.g. ::ef13:2.1.7.100. This is a valid ipv6 notation! If this space ever got too tight one could open another one of the available hex groups and use two hex group prefixes. But I think when this happens, a lot of configurations would break because they'd assume that only 48 bits are used of the total 128. To prevent router,switch,firewall, etc. vendors from putting any such assumptions into their devices, using the full 128 bits from the start is a good option.