IPv4 allocates a whole block of IPv4 loopback addresses,
127.0.0.0/8. IPv6, in contrast, only allocates a single loopback address,
This seems surprising that IPv6 would be so stingy in its allocation of loopback address(es), since overall it provides a huge increase in address space.
In IPv4 I've found it useful to be able to use multiple loopback addresses. E.g. for certain complex DNS set-ups, I can install two local DNS servers, one on
127.0.0.1:53, and another on
127.0.1.1:53. (It's a bit easier to refer to the different servers using different IP addresses, rather than using non-standard port numbers on a single IP address.) I can't use this strategy on IPv6 with only one loopback address.
What good rationales might there have been for only allocating a single loopback address for IPv6, unlike for IPv4?