What problem did CIDR solve in IPv4 addressing?

• A. It increased the maximum address length from 32 to 128 bits.
• B. It enabled variable-length subnet masks and route aggregation to reduce routing table size.
• C. It provided automatic address assignment via DHCP.
• D. It defined a loopback range for testing.

Answer: (B)

CIDR stands for Classless Inter-Domain Routing, and it solves two practical problems that came with IPv4: address waste and huge routing tables. Before CIDR, IPv4 used fixed classful masks (A, B, C), which forced many networks to use more addresses than they needed or to be split into many small, separate networks. That wasteful rigidity made address allocation inefficient. CIDR lets you use variable-length subnet masks, so you can tailor the number of host bits to actual needs and avoid waste.

But the bigger win is route aggregation. With CIDR, many adjacent subnets can be summarized into a single, larger prefix. Internet routers can advertise and store far fewer routes because a single aggregated prefix covers many individual networks. This dramatically reduces the size of global routing tables and makes routing scalable as the network grows.

The other options don’t reflect what CIDR does: increasing address length to 128 bits belongs to IPv6, not CIDR; automatic address assignment via DHCP is a separate protocol; and a loopback range is just a testing address, unrelated to CIDR’s routing and addressing mechanism.