The subnet mask is used to specify which portion of the unicast IPv4 address defines the network value and which portion of the unicast IPv4 address defines the unique host value. The subnet mask can be shown as dotted decimal, as with 255.255.255.0, or as slash notation, as in /24.

The slash notation is easier to use as it defines the same information in a more convenient format. If you look at the Class A default (or natural) mask or 255.0.0.0, you can say that 255 is 8 ones (converting a decimal 255 to binary yields 1111 1111).

Slash 8 simply means there are 8 ones in the subnet mask (or 255.0.0.0). By using 255, you are selecting the octet or octets (or, in some cases, the piece of an octet) used to identify the network address.

For example, in the Class B network address 192.168.2.1, with the default subnet mask for a Class B space being 255.255.0.0, then 192.168 is the network address and 2.1 is the unicast host address.