CIDR (Classless Inter-Domain Routing)

CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing internet traffic. It was introduced in 1993 to replace the older system of classful networking, providing more efficient use of IP address space and allowing for better routing scalability.

Key Concepts of CIDR:

1. IP Address Representation:

   • CIDR notation combines an IP address with a subnet mask. The format is IP_address/Prefix_length.
   • Example: 192.168.1.0/24
     • 192.168.1.0 is the network address.
     • /24 indicates the subnet mask, specifying that the first 24 bits are used for the network portion of the address, and the remaining bits are for host addresses.

2. Subnet Mask:

   • The prefix length (e.g., /24) replaces the traditional classful subnet masks like 255.255.255.0.
   • It defines how many bits of the IP address are dedicated to the network part, leaving the rest for the host.

3. Efficient IP Allocation:

   • CIDR allows for more flexible and efficient allocation of IP address blocks, unlike classful addressing, which was rigid (with classes A, B, and C).
   • For example, instead of allocating an entire class C network (which has 256 addresses), CIDR allows for allocating only a fraction of it, say 128 addresses, using a /25 subnet.

4. CIDR Blocks:

   • CIDR enables the use of various subnet sizes, such as /22 for 1,024 addresses, /23 for 512 addresses, etc., instead of being limited to the fixed class-based ranges.

5. Route Aggregation:

   • CIDR also enables route aggregation, which reduces the size of routing tables on the internet. Multiple networks can be aggregated into a single routing entry, improving routing efficiency.

Example of CIDR Usage:

• 10.0.0.0/8 — This refers to a network that includes all IP addresses from 10.0.0.0 to 10.255.255.255, which can host up to 16,777,216 addresses (2^24).
• 192.168.1.0/24 — This subnet contains 256 addresses (from 192.168.1.0 to 192.168.1.255), and it is typically used in small networks.

Advantages of CIDR:

• Efficient IP address allocation: More flexible than class-based allocation.
• Reduced routing table size: Thanks to route aggregation, multiple routes can be grouped into a single entry.
• Conservation of IP address space: More precise subnetting means less waste of IP addresses.

CIDR has become a standard method for assigning and managing IP addresses, both for IPv4 and IPv6 networks.