A Subnet Mask is a 32-bit number that divides an IP address into network and host portions. It helps routers and switches determine which part of the address identifies the network and which part identifies the specific device within that network. This is crucial for efficient data routing and communication across the internet. By using a subnet mask, network administrators can create smaller, manageable networks within a larger one, enhancing security and performance. Common subnet masks include 255.255.255.0 and 255.255.0.0, which allow for different numbers of devices to be connected within a subnet.