A subnet mask is a numerical label used in computer networks to divide an IP address into two parts: the network portion and the host portion. It helps identify which part of the address refers to the network and which part refers to individual devices within that network. Subnet masks are typically written in the same format as IP addresses, using four sets of numbers separated by periods. For example, a common subnet mask is 255.255.255.0, which indicates that the first three sets of numbers represent the network, while the last set identifies individual devices. This allows for efficient routing of data and helps manage network traffic by grouping devices into smaller, manageable segments.