The 255.255.255.0 subnet mask is the most common subnet mask used on computers connected to Internet Protocol (IPv4) networks. In addition to its use on home network routers, you may also see this mask on professional networking certification exams such as the CCNA.

Subnets act as virtual fences, dividing a block of IP addresses into smaller units. This practice relieves network congestion and allows granular access across subnets. A subnet mask identifies individual subnets.

Traditional subnetting worked with class networks that divided IP addresses into one of five classes (Class A/B/C/D/E) based on the value of the IP address number.

The subnet mask 255.255.255.0 is converted to a 32-bit binary value: