A subnet mask is a number that helps divide an IP address into two parts: the network part and the host part. It tells devices on a network which portion of the IP address identifies the network and which part identifies individual devices, or hosts. For example, in the IP address 192.168.1.1 with a subnet mask of 255.255.255.0, the first three numbers (192.168.1) represent the network, while the last number (1) identifies the specific device. Subnet masks are essential for efficient network management and communication. They allow multiple networks to coexist on the same physical network, improving security and performance. By using subnetting, organizations can better allocate their IP addresses and control traffic flow, making it easier to manage large networks