What is the difference between classful addressing and CIDR?

What is the difference between classful addressing and CIDR?

• A. CIDR is deprecated
• B. Classful uses fixed default masks by class; CIDR uses flexible subnet masks based on prefix length
• C. CIDR uses fixed default masks; classful uses variable
• D. Classful and CIDR both use the same default masks

Answer: (D)

Classful addressing fixes the network size by the address class, using a default subnet mask for each class (for example, a class A address assumes 255.0.0.0, class B uses 255.255.0.0, class C uses 255.255.255.0). This means the network portion is determined by the class, and all subnets within that class essentially follow that predetermined size. CIDR changes that by letting you specify a prefix length instead of sticking to a class default. A prefix like /24 means the first 24 bits are the network, and the remaining bits are for hosts. You can have many different subnet sizes (such as /20, /26, /30, etc.), and you can aggregate routes more efficiently. So the key difference is fixed, class-based masks versus flexible, prefix-length masks that CIDR uses to tailor subnet sizes and enable route aggregation.

Classful addressing fixes the network size by the address class, using a default subnet mask for each class (for example, a class A address assumes 255.0.0.0, class B uses 255.255.0.0, class C uses 255.255.255.0). This means the network portion is determined by the class, and all subnets within that class essentially follow that predetermined size.

CIDR changes that by letting you specify a prefix length instead of sticking to a class default. A prefix like /24 means the first 24 bits are the network, and the remaining bits are for hosts. You can have many different subnet sizes (such as /20, /26, /30, etc.), and you can aggregate routes more efficiently.

So the key difference is fixed, class-based masks versus flexible, prefix-length masks that CIDR uses to tailor subnet sizes and enable route aggregation.