Classful addressing is a method used in computer networking to allocate IP addresses into distinct classes based on their leading bits. The primary classes are Class A, Class B, and Class C, each designed for different network sizes. For example, Class A supports large networks with many hosts, while Class C is suitable for smaller networks. In classful addressing, the subnet mask is implied by the class of the IP address, which simplifies routing but can lead to inefficient use of IP addresses. This method has largely been replaced by Classless Inter-Domain Routing (CIDR), which allows for more flexible and efficient allocation of IP addresses.