What is the primary purpose of a subnet mask?

The primary purpose of a subnet mask is to divide an IP address into network and host portions. This is fundamental for managing and organizing IP addresses within a network. A subnet mask helps distinguish which part of the IP address refers to the network itself and which part designates the specific device or host within that network.

When used in conjunction with an IP address, the subnet mask allows routers and switches to determine the routing of data packets across a network. By using a subnet mask, devices can identify whether an IP address belongs to a local network segment or if it is outside of that segment, necessitating a route through a gateway.

The other options, while related to network functionality, do not accurately reflect the role of a subnet mask. For instance, defining network speed and bandwidth is more about the physical layer of networking, encryption pertains to securing communications rather than the structure of addressing, and monitoring traffic is a separate function that does not involve the basic definition and division of IP addresses.