A router has route entries for 172.31.0.0/16, 172.31.0.0/24, and 172.31.0.0/25. A packet to 172.31.0.1 arrives—which route is chosen?

Routers use the longest prefix match rule to determine the best path for a packet. This means the route with the most specific match (the highest subnet mask value) to the destination IP address is preferred. For the destination 172.31.0.1, all three listed routes match. However, 172.31.0.0/25 has a /25 subnet mask, which is more specific than /24 or /16. Therefore, the router will choose the 172.31.0.0/25 route. The default gateway (0.0.0.0/0) is only used when no more specific route exists.