What is CIDR?

CIDR stands for Classless Inter-Domain Routing. It is a way of assigning and managing IP addresses and routes in IP networks. CIDR, introduced in 1993, replaced the old system of classful network addressing, which split IP addresses into fixed and predefined classes (Class A, Class B, and Class C).

CIDR allows for flexible allocation of IP address space by using variable-length subnet masking (VLSM). With CIDR, IP addresses are written as a network prefix followed by a slash (“/”) and a number indicating the number of significant bits in the prefix.

For example, means a network with a 16-bit prefix, indicating the first 16 bits are the network part and the remaining bits are the host part.

CIDR has become the standard addressing scheme used in modern IP networks, including the Internet. It allows for efficient allocation of IP addresses and more flexible network design, enabling organizations to optimize their IP address usage and streamline routing operations.

CIDR offers several benefits over classful addressing:

1. Efficient address allocation

CIDR enables to assign IP addresses in a more granular and efficient way, eliminating the need for fixed-sized blocks. It allows for allocating smaller subnets, saving IP address space.

2. Aggregation

CIDR enables route aggregation, where multiple smaller address blocks can be merged into a single larger block. This reduces the number of routing table entries and improves the efficiency of routing.

3. Flexibility

CIDR allows for subnetting, which means dividing a larger IP address block into smaller subnets. Subnetting provides flexibility in network design, allowing organizations to assign IP addresses based on their specific needs.

4. Simplified routing

CIDR simplifies routing by enabling supernetting, which means summarizing multiple contiguous network prefixes into a single, larger prefix. This reduces the size of routing tables and improves the scalability and performance of routers.

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