Classful subnetting is an older method of subnetting IP addresses that follows the traditional class-based IP address structure, which includes Class A, Class B, and Class C networks. In classful subnetting, the subnet mask is predetermined based on the class of the IP address. Each class has a default subnet mask, and subnetting is not as flexible as in classless subnetting (CIDR - Classless Inter-Domain Routing).

Classful subnetting was widely used in the early days of the internet when IP addresses were allocated based on the class of the network. However, it has limitations, such as inefficient use of IP address space and challenges in accommodating different network sizes.

Classful subnetting is rarely used today due to its limitations and the adoption of CIDR, which allows for more flexible subnetting by using variable-length subnet masks (VLSM). CIDR enables the allocation of IP addresses based on actual network requirements rather than predefined classes. Modern networking and internet infrastructure predominantly rely on CIDR for subnetting, offering greater efficiency and scalability in IP address allocation.