What is the primary function of a subnet mask in networking?

The primary function of a subnet mask is to enable devices to find their subnet. In networking, a subnet mask is used to determine which portion of an IP address refers to the network and which part refers to the host. By applying the subnet mask, devices can understand their local network boundaries and communicate effectively within those boundaries.

When a device wants to send data to another device, it uses the subnet mask to check if the destination device is within the same local subnet or if it needs to send the data to a router for forwarding outside the local network. This capability is essential for routing design and efficient communication, ensuring that data packets reach their intended destinations accurately.

The other options provide functionalities that are not the primary role of a subnet mask. The function related to restricting access to a network pertains more to security measures such as firewalls or access control lists. Data encryption is a method used for securing information during transmission, rather than a task performed by a subnet mask. Lastly, while subnetting can contribute to better network organization, it does not directly improve network speed; rather, it structures how devices communicate within their defined segments.