In IP addressing, what is CIDR (Classless Inter-Domain Routing) and why was it introduced to replace traditional classful addressing?

Introduction / Context:
Classless Inter-Domain Routing (CIDR) is a key concept in modern IP addressing and routing. It replaced the older classful system of fixed Class A, B, and C networks. Interview questions about CIDR typically check whether you understand variable-length prefixes, slash notation (such as /24), and the reasons for moving away from classful addressing, such as address conservation and smaller routing tables.

Given Data / Assumptions:

• We are working with IPv4 addressing.
• Classful addressing originally used fixed default masks for Class A, B, and C networks.
• IPv4 address space is limited and must be used efficiently.
• Routers maintain routing tables with prefixes and next hops.

Concept / Approach:
CIDR introduced classless addressing, where networks are identified by arbitrary length prefixes instead of fixed classes. Instead of talking about "Class C 192.168.1.0", we now describe networks using notation like 192.168.1.0/24, where /24 indicates that the first 24 bits are the network portion. Service providers can allocate address blocks with sizes that closely match customer needs, such as /28 or /29, instead of entire Class C blocks. Routers can also aggregate multiple contiguous prefixes into a single summary route, reducing the size of global routing tables and improving scalability of the internet.

Step-by-Step Solution:
Step 1: Define CIDR as Classless Inter-Domain Routing, a classless addressing scheme for IP networks. Step 2: Explain that CIDR uses prefix length notation (for example /22, /24, /30) instead of fixed Class A, B, or C defaults. Step 3: Describe how this allows more precise allocation of address space to organizations, conserving scarce IPv4 addresses. Step 4: Highlight that routers can group several contiguous networks into a single summarized prefix, reducing routing table entries. Step 5: Conclude that CIDR was introduced to slow IPv4 exhaustion and improve routing scalability on the global internet.

Verification / Alternative check:
Reviewing BGP routing tables from internet exchange points shows that nearly all routes are expressed in CIDR notation with various prefix lengths instead of rigid Class A, B, or C networks. Documentation from standards bodies confirms that CIDR was deployed in the 1990s to combat rapidly growing routing tables and address consumption. IP subnetting exercises in training materials consistently use CIDR prefix lengths to represent subnets and supernets, validating its central role in modern IP networking.

Why Other Options Are Wrong:
Option B describes the old classful system that CIDR explicitly replaced. Option C mischaracterizes CIDR as an encryption protocol, which it is not. Option D confuses addressing with cabling standards; CIDR operates at the network layer and has nothing to do with wire pairs or physical media.

Common Pitfalls:
Learners sometimes think that terms like Class C still strictly apply in modern networks; in reality, CIDR and subnetting have largely replaced class based thinking. Another pitfall is misunderstanding prefix lengths and accidentally allocating too large or too small subnets, leading to inefficient address usage or overlapping routes. A solid grasp of CIDR notation and its benefits is crucial for designing scalable, efficient IP networks.

Final Answer:
CIDR is a classless IP addressing method that uses variable-length network prefixes (slash notation) to allocate address blocks efficiently and reduce routing table size.