RFC 8215 “Local-Use IPv4/IPv6 Translation Prefix” was recently published, reserving the IPv6 prefix 64:ff9b:1::/48 for local use within domains enabling IPv4/IPv6 translation mechanisms.
This allows the coexistence of multiple IPv4/IPv6 translation mechanisms in the same network, without requiring the use of a Network-Specific Prefix assigned from an allocated global unicast address space.
The well-known prefix 64:ff9b::/96 was originally reserved by RFC6052 for IPv4/IPv6 translation, but several new translation mechanisms such as those in RFCs 6146 and 7915 have subsequently been defined that target different use cases. It’s therefore possible that a network operator may wish to make use of several of these simultaneously, hence why a larger address space has been defined to accommodate this.
The shortest translation prefix being deployed in a live network was observed as being a /64, hence /48 was chosen as being on a 16-bit boundary whilst being able to accommodate multiple instances of /64.