What system replaced the classful IPv4 addressing system?

The classful IPv4 addressing system was replaced by Classless Inter-Domain Routing (CIDR) primarily to address the issues of address space exhaustion and to enable more efficient use of IP addresses. In the classful system, IP addresses were divided into fixed classes (A, B, C) based on predefined ranges, which often led to inefficient allocation of IP addresses and wasted address space.

CIDR introduced a more flexible approach by allowing IP addresses to be allocated without being restricted to these classes, thereby enabling variable-length subnet masking (VLSM). With CIDR, addresses can be expressed in a format like '192.168.1.0/24', where '/24' indicates the subnet mask used, allowing for more precise control over how many addresses are assigned within a network. This flexibility leads to better utilization of the available address space and supports the growing number of devices requiring IP addresses.

As a result, CIDR became essential for accommodating the expansion of the Internet and has facilitated the efficient aggregation of IP routes, which helps in reducing the size of routing tables and improving routing efficiency on the Internet.