Classless Inter Domain Routing. Currently subnets are defined by the number of binary bits they have in common. This replaces the older subnet class system. Each octet of the IP address can be broken down into two hexadecimal digits; it takes 8 bits to represent each pair of hexadecimal digits. So a class A subnet which shares the first octet would be an 8 Bit subnet. A class B would be a 16 Bit subnet. A class C subnet would be a 24 bit subnet.
If your ISP provides you with a subnet you will likely have many more bits in common. For example, if you were given a 29 bit subnet, you would have 8 IP addresses, an Identifier address, 6 useable IP addresses, and a broadcast IP address.
CIDR in Wikipedia