Final answer:
With 'no ip classless' enabled and no matching child route, a router will drop the packet and send an ICMP unreachable message to the packet's source.
Step-by-step explanation:
When no ip classless is enabled on a router and it receives a packet for which it does not have a specific child route, the router's behavior is directed by the concept of classful routing. In this scenario, if the router does not find a matching child route, it cannot fall back to a longer matching route. Consequently, the router will not forward the packet to a default route or perform a recursive lookup. Instead, the router will drop the packet and send an ICMP unreachable message to the source of the packet, indicating that the destination is not reachable.