Or you simply rewrite the first 48 bits when the packet traverses the border into the destination site with the same value the packet had prior to traversing the border of the source site.If the prefix is mutable en route, it doesn't matter that it's globally unique: the end points can't use it as part of any checksum, so either you have to add HIP, or the 64 bit suffix has to be unique enough for checksum purposes.