What is the purpose of a subnet mask?

The purpose of a subnet mask is to divide an IP address into network and host portions. In networking, an IP address is made up of two main components: the network part identifies which network the device belongs to, and the host part identifies the specific device (or host) within that network. The subnet mask works alongside the IP address to determine how to interpret this division.

When a device sends or receives data, the subnet mask helps to determine whether the destination IP address is on the same local network or if the data needs to be sent to a different network. This function is critical for routing purposes, as it informs the device how to process IP addresses.

Given this understanding, the other choices relate to separate functions that are not the primary purpose of a subnet mask. For example, securing data transmission and encrypting data packets focus on cybersecurity rather than addressing and routing. Identifying unauthorized network traffic is also a function related to network security but does not pertain to the mechanics of how IP addresses are structured. Therefore, the correct answer accurately reflects the core function of a subnet mask in networking.