CIDR enables which feature that classful addressing does not support efficiently?

• A. The ability to allocate IPs without any subnetting
• B. Automatic private IP addressing when DHCP is unavailable
• C. The ability to create subnets of arbitrary sizes (variable-length subnet masks)
• D. Fixed-length subnets with no variation

Answer: (C)

CIDR’s main idea is flexible subnet sizing through variable-length subnet masks. By allowing prefix lengths to be any value, CIDR lets you carve networks to match exactly how many hosts you need, instead of sticking to fixed sizes tied to a address class. This makes address use much more efficient and also enables route aggregation. For example, you can allocate a /26 for 62 usable addresses when you need about 60 hosts, instead of forcing a full /24 or larger. In contrast, classful addressing fixes the subnet size based on the address class, which wastes space or forces awkward, oversized networks.

So the feature CIDR enables is creating subnets of arbitrary sizes (variable-length subnet masks). The other options don’t describe this capability: CIDR isn’t about subnetting being unnecessary, nor about DHCP-related automatic private addressing, nor about fixed-length subnets.