IPv4 addresses are divided into five classes. Each IP address class has a different maximum number of IPs that exist within it.
|Class||First Octet Range||Default Subnet Mask||Maximum Number of Hosts Per Network|
|A||1-127||255.0.0.0||16,777,214 Hosts (2^24-2)|
|B||128-191||255.255.0.0||65,534 Hosts (2^16-2)|
|C||192-223||255.255.255.0||254 Hosts (2^8-2)|
Class A IPs start with 1-127. Keep in mind, though, that 127 is reserved for localhost. The remaining three octets are used for the host id. Class B IPs start with 128-191 and use the first and second octets for the network id. Class C begins with 192-223 and use the first three octets for the network id. Class D and class E are not generally used for setting up networks. Class D is Multicast and class E is experimental.