IPv4 is based on a 32-bit address scheme that could in theory enable a total of 4 billion hosts (exactly 4,294,967,296) on the whole Internet. However, this 32-bit scheme was originally divided into five hierarchical classes managed by the Internet Assigned Numbers Authority (IANA). The first three classes (A, B, and C) are available as globally unique unicast IP addresses. These classes were assigned to the requesters with a fixed prefix length using different netmask values. A netmask is consecutive series of bits preset to 1 designed to “mask” the network part of an IP address.
Table 1-1 shows the five classes of IPv4 addresses, along with their associated ranges and network masks.