The only way to do what you want is to effectively have a variable
length address. While there were a few crazy advocates of this many
years ago, they were shouted down.
variable length addresses are a better idea than it appears at first
glance. they do bring certain difficulties with them, especially when
trying to do fiber-speed switching in hardware. it's hard to switch on
arbitrary bit boundaries and it's harder to switch in hardware using
prefixes of arbitrary length. but there are ways to deal with these
problems. nobody should expect core routers to pay attention to less
significant bits of a variable length address any more than they pay
attention to the less significant bits of a fixed length address, and
just because addresses are variable length doesn't mean that there
cannot be a maximum length of a prefix advertised in BGP.
overall it strikes me as a hell of a lot better than NAT.
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