Classless Inter-Domain Routing also known as the CIDR notation is an alternate mode of representing a subnet mask. It is a count of the number of network bits set to 1 in the subnet mask. It is preceded by a hash sign “/”. To answer your question, the subnet 255.255.255.0 would be represented as /24 and in binary would be written as 11111111 11111111 11111111 00000000