has a classfull C somewhere on the net. As for filtering or
the entire block because it is reserved for that matter, who is to say
that with the address space thinning out that the IANA won't start
allocating it? Some poor provider is going to be route filtered on a
totally legitimate block if we use your reasoning.

Its called "Network Management" and not "Network forget about it" for
a reason.

I'm making no statement about filtering by prefix length. Unassigned
is true whether it is a unassigned traditional Class A address
or a unassigned Class ? address.

IANA has been exceptionally careful to document and put in a public
place all its assignments. A careful network manager could check this
public information on a regular basis and use it to verify the information
they are hearing from others.

Address hijacking is a major and growing problem. There are two ways
to stop it, and I would suggest you need to do both. You can either
trust everyone will only announce their own routes properly, or you can
check the routes you recieve that they have in fact been delegated to
the person announcing it. Or in the case of, delegated at all.

There are extremes, you can check every single prefix or you can do
no checking at all. Or you can do some intermediate level of checking
such as filtering 'Martian' addresses, or addresses unassigned or reserved
by IANA.