IP addresses are generally categorized as belonging to one of three different classes. IPs with the first byte in the 0-127 range are classified as Class A IPs, those in the 128-191 range as Class B IPs and those in the 192-223 range as Class C IPs.
Each IP class can be subnetted into smaller ones to facilitate the distribution and routing of networks on the Internet. This calculator can be used to determine the number of IP addresses available in a given class or subnet.