How subnet mask is used by a router?

A subnet mask is used by a router to determine which part of an IP address identifies the network and which part identifies the host. When a router receives a packet, it applies the subnet mask to the destination IP address to ascertain if the destination is within the same local network or if it needs to route the packet to a different network. This helps in efficient routing and ensures that data packets are sent to the correct destination. Additionally, it allows routers to manage and segment network traffic effectively.