What is a subnet mask used for in networking?

A subnet mask is essential in networking as it defines the boundary between the network portion and the host portion of an IP address. This delineation is vital for routing and addressing within IP networks.

When an IP address is assigned to a device, the subnet mask helps determine which part of the address identifies the specific network (or subnet) the device belongs to and which part can identify the individual device (or host) within that network. This allows routers to efficiently forward packets to their destinations and aids in proper network organization, enabling communication between devices on the same local network as well as with devices on different networks.

The other choices do not serve the purpose of defining network and host portions: data rates are related to bandwidth and speed, encryption pertains to securing data, and monitoring network access deals with controlling who can access the network. Thus, the role of the subnet mask is pivotal for proper IP network management and architecture.