How does CIDR differ from classful addressing?

• A. CIDR uses a prefix length (/n) to define subnets regardless of class; classful addressing uses fixed default masks and is limited by historical classes.
• B. CIDR uses a fixed default mask by class; classful uses variable length prefixes.
• C. CIDR is older and more rigid than classful addressing.
• D. CIDR cannot support subnetting.

Answer: (A)

CIDR defines networks using a prefix length, written as /n, that specifies how many bits are part of the network address. This works independently of any class and lets you carve subnets of many different sizes, as well as group several networks into a single route for efficient routing. In contrast, classful addressing relies on fixed default masks tied to the address class (A, B, or C), such as 255.0.0.0, 255.255.0.0, or 255.255.255.0, which rigidly dictates subnet size and can waste addresses.

So the key distinction is that CIDR uses a flexible prefix length to define subnets regardless of class, while classful addressing uses fixed class-based masks. This flexibility is what enables both precise subnetting and route aggregation.