AsIPPrefix returns a as a netaddr.IPPrefix.
If a has Bits of -1, indicating an invalid bits,
the returned IPPrefix will have Bits of 255.
AsIPPrefix and AsPrefix do not
round trip for invalid Bits values.
AsPrefix returns a as a netip.Prefix.
If a has an invalid Bits value,
the returned Prefix will have Bits of -1.
AsIPPrefix and AsPrefix do not
round trip for invalid Bits values.