Computers or devices need to know where you drew that line. That’s where subnet masks come in. These masks are a series of ones and zeros that tell the device which part is the network, and which is the host. In the example below, the subnet mask is show in the red box, first in binary, then in decimal.

One thing to note, subnet masks weren’t necessary when we had IP address classes. You could determine the subnet without a mask. But when subnets came into play, we needed masks to specify where the line was drawn. If you stuck to classful boundaries, it became a default subnet mask.

One thing to note, subnet masks weren’t necessary when we had IP address classes. You could determine the subnet without a mask. But when subnets came into play, we needed masks to specify where the line was drawn. If you stuck to classful boundaries, it became a default subnet mask.