How do classful addressing and CIDR differ?

• A. CIDR uses fixed default subnet masks; classful uses variable-length prefixes.
• B. Classful uses fixed default subnet masks per class; CIDR uses variable-length prefixes for flexible subnetting.
• C. Classful is used for IPv6; CIDR is for IPv4.
• D. Classful allows VLSM; CIDR does not.

Answer: (B)

The main idea here is how networks are sized and represented. Classful addressing fixes the subnet size based on the address class: a class A network uses a /8 mask, class B uses a /16, and class C uses a /24. This creates rigid boundaries that often waste addresses or force suboptimal splits because you must fit networks into those default blocks. CIDR changes that by allowing a prefix length to be specified after the address, like /23, /20, or /28. That means networks can be sized exactly to the number of hosts needed, improving address utilization and enabling flexible subnetting. CIDR also supports route aggregation, which helps keep routing tables smaller by summarizing many networks under a single prefix. While IPv6 uses a prefix-length concept too, CIDR’s value is in providing variable-length prefixes for IPv4 as well. So the correct description is that classful uses fixed default masks per class, whereas CIDR uses variable-length prefixes for flexible subnetting. The statements that CIDR uses fixed masks or that classful supports VLSM (and the claim that CIDR is only for IPv4) aren’t accurate.