Define the term "subnet mask" and explain how it is used to divide an IP address into network and host portions.

A subnet mask is a 32-bit number that masks an IP address and divides the IP address into network address and host address. The subnet mask is made up of 32 bits, and it can be used to identify the network ID and the host ID of an IP address.

Here's how it works:

1. An IP address is a unique identifier for a device on a network. It is made up of two parts: the network address and the host address. The network address identifies the specific network on which the device is located, while the host address identifies the specific device on that network.

2. The subnet mask is used to divide the IP address into these two parts. It does this by applying a binary AND operation to the IP address and the subnet mask.

3. In the subnet mask, all the network bits are set to 1, and all the host bits are set to 0. For example, for a Class C IP address, the subnet mask would be 255.255.255.0. In binary, this would be 11111111.11111111.11111111.00000000.

4. When the binary AND operation is applied to the IP address and the subnet mask, the result is the network address. The remaining bits represent the host address.

5. By changing the subnet mask, you can change how the IP address is divided between network and host. This is useful for subnetting, or dividing a network into smaller subnetworks.

6. Subnetting can help improve network performance and security. By creating smaller subnetworks, you can reduce network traffic and create separate, smaller networks for different purposes.