In our example, there are hosts behind the firewall with host identifiers :1, :2, and :3. If the prefixes of those hosts are translated to a prefix that exists beyond the firewall, and if those devices also have host identifiers :1, :2, and :3, because the host identifier portion of the address remains unchanged, the resulting translated address would belong to the existing device, and an addressing conflict would result. In order to avoid a conflict with overlapping host identifiers, NPTv6 does not translate addresses that it finds it its ND cache.