What is CIDR notation and how does it relate to subnet masks?

• A. CIDR is a security protocol.
• B. CIDR uses 8-bit prefixes only.
• C. CIDR and subnet masks are unrelated.
• D. CIDR uses a prefix length (e.g., /24) to define the subnet mask.

Answer: (D)

CIDR notation is a compact way to describe how many leading bits in an IPv4 address are used to identify the network. In IPv4 there are 32 bits total, and the subnet mask marks which bits define the network portion versus the host portion. CIDR uses a slash followed by a number to show that boundary. For example, a prefix length of 24 means the first 24 bits are the network, and the remaining 8 bits define hosts. That /24 boundary corresponds to the dotted-decimal subnet mask 255.255.255.0. Other examples: /16 maps to 255.255.0.0, /28 maps to 255.255.255.240, and so on. CIDR is just a concise way to express the same idea as a subnet mask, and it lets you flexibly carve networks of varying sizes instead of sticking to fixed classful sizes.