What does a subnet mask do?

The role of a subnet mask is to divide an IP address into two parts: the network part and the host part. This division allows devices to determine whether a given IP address is on the same local network or if the data must be routed to a different network.

When an IP address is assigned to a device, the subnet mask works in conjunction with that IP address to define which portion of the address refers to the network (where devices on the same network can communicate directly) and which part identifies the specific device (or host) within that network. For example, in a typical subnet mask like 255.255.255.0, the first three octets (255.255.255) represent the network part, while the last octet (0) indicates individual hosts on that network. This structure is crucial for effective routing and network communication.

The other choices do not accurately reflect the function of a subnet mask. While optimizing network performance and identifying connected devices are important network management aspects, they are not the primary purpose of a subnet mask. Likewise, encryption is a separate concern in network security and does not pertain to the role of a subnet mask.