Internet addresses are allocated by the InterNIC, the organization that administers the Internet. These IP addresses are divided into different classes. The most common of them are classes A, B, and C. Classes D and E exist but aren’t used by end-users. You can identify the class of an IP address by looking at its first octet. IP classes A, B, and C provide unicast addresses for networks of three different network sizes. Class D is for multicast networking and the class E address range is reserved for future or experimental purposes. The main difference is that Class A IP addresses are used for huge networks to support up to 16 million hosts, Class B IP addresses are used for medium and large-sized networks in enterprises and organizations to support up to 65,000 hosts and Class C addresses are most used in small business and home networks to support up to 256 hosts. You can find out more about the different features of each IP class via Wikipedia or this SeekaHost IP classes guide.