Which of the following best describes CIDR (Classless Inter-Domain Routing)?

The correct answer is Option 1) Allocating IP address in variable sized blocks regardless of class.

Key Points

• CIDR (Classless Inter-Domain Routing) was introduced in 1993 to replace the outdated classful IP addressing system.
• Under classful addressing:
  • Class A used a default subnet mask of /8 (e.g., 255.0.0.0)
  • Class B used /16
  • Class C used /24
  • This rigid structure led to significant wastage of IP addresses.
• CIDR removed the dependency on fixed classes and allowed the assignment of IP addresses in variable-sized blocks — this is known as Variable Length Subnet Masking (VLSM).
• CIDR addresses are written in the format: IP_address/prefix_length, where prefix_length indicates the number of bits used for the network portion.
• Example: 192.168.10.0/22 means:
  • The first 22 bits are for the network ID
  • The remaining 10 bits are for host IDs
  • This allows for 1024 IP addresses (2¹⁰) in this block

Additional Information

• Why CIDR is better than classful addressing:
  • Efficient IP allocation: Blocks can be allocated exactly based on need (e.g., /29 for 6 hosts instead of wasting a full Class C /24)
  • Scalability: Supports hierarchical IP address allocation to reduce routing table entries
  • Aggregation: Multiple CIDR blocks can be summarized into a single route entry (route aggregation or supernetting)
• Incorrect Options:
  • Option 2: Refers to the classful model, which CIDR was explicitly designed to replace.
  • Option 3: CIDR is part of IP addressing and routing; DHCP is for dynamic IP assignment.
  • Option 4: CIDR and subnetting are related, but CIDR is more flexible and comprehensive than traditional subnetting.
• Real-world example: An ISP may assign a customer a /29 CIDR block, giving them 6 usable IP addresses (instead of 254 in a /24 block), which saves a lot of IP space.