What is a subnet mask and how is it used?

A subnet mask is a 32-bit number, typically expressed in dotted-decimal notation (e.g., 255.255.255.0), used in conjunction with an IP address to determine the network portion and the host portion of the address in an IP network.

In simpler terms, a subnet mask helps devices on a network determine which part of an IP address identifies the network and which part identifies the specific device on that network.

Here's how it works:

1. Network Portion: The subnet mask identifies which bits in an IP address belong to the network portion. These bits are usually represented by a series of consecutive 1s in the subnet mask. For example, in the subnet mask 255.255.255.0, the first 24 bits (or the first three octets) are dedicated to identifying the network.

2. Host Portion: The remaining bits in the IP address (those not dedicated to the network portion) are used to identify individual devices on the network. These bits are usually represented by a series of consecutive 0s in the subnet mask.

3. Application: When a device wants to send data to another device on the same network, it uses the subnet mask to determine if the destination IP address is on the same network as itself. If it is, the device sends the data directly to the destination device. If the destination IP address is on a different network, the device sends the data to the appropriate router or gateway, which will then forward the data to the correct network.

In summary, a subnet mask helps devices determine which part of an IP address identifies the network and which part identifies the specific device, allowing for efficient routing of data within a network.