In CIDR notation, the subnet mask 255.255.255.0 is written as /24.
An IPv4 address contains 32 bits. This mask sets the first 24 bits to 1—eight in each of the first three octets—and leaves the final eight bits for host addresses. The equivalent binary mask is 11111111.11111111.11111111.00000000.
A /24 network contains 256 total address values, from the network address through the broadcast address. In ordinary IPv4 subnetting, that leaves 254 assignable host addresses because the first and last values have special purposes. For example, 192.168.1.0/24 covers 192.168.1.0 through 192.168.1.255.
A common mix-up is confusing the prefix length with the number of host bits: /24 means 24 network bits, not 24 hosts. By contrast, /16 uses mask 255.255.0.0, /8 uses 255.0.0.0, and /32 identifies one individual IPv4 address.