What does CIDR mean?

Question

What is CIDR (Classless Inter-Domain Routing), and how does it function in modern networking? Please explain its purpose, advantages, and how it differs from the traditional classful network addressing.

Accepted Answer

CIDR, or Classless Inter-Domain Routing, is a method for allocating IP addresses and routing IP packets. It was introduced in 1993 to replace the older system of classful network addressing. CIDR plays a crucial role in the efficient management and use of IP address space, which is vital given the finite number of available IP addresses.

### Purpose of CIDR:

1. **IP Address Aggregation**: CIDR allows for the aggregation of IP addresses into a more streamlined, hierarchical structure, known as supernetting. This approach reduces the size of routing tables, improving the efficiency of routers and supporting global Internet scalability.

2. **Flexible Network Design**: Unlike classful addressing, which rigidly defines address boundaries (Class A, B, or C networks), CIDR allows network administrators to divide an IP address space into variable-sized subnets. This flexibility enables a better fit between the network size and its address allocation requirements.

3. **Conservation of IP Addresses**: CIDR contributes to IP address conservation by allowing more efficient distribution and reducing wasted addresses. This is especially valuable given the limited address space in the IPv4 protocol.

### How CIDR Functions:

CIDR represents IP addresses with a suffix that indicates the number of bits in the network prefix. For example, the CIDR notation `192.168.0.0/24` denotes an IP address with a 24-bit prefix, leaving 8 bits for host addresses. This notation supports varying network sizes, enabling the creation of networks that do not fit neatly into the traditional class boundaries.

### Advantages of CIDR:

1. **Efficient Routing**: With CIDR, large blocks of addresses can represent routes in routing tables, minimizing the number of entries and simplifying route management.

2. **Address Conservation**: CIDR's ability to create subnetworks (or subnets) with different sizes means fewer unused addresses within a given network.

3. **Scalability**: The system supports the expansion of networks by allowing for hierarchical addressing and efficient route aggregation across the Internet.

### Differences from Classful Network Addressing:

- **Variable-Length Subnet Masks (VLSM)**: While classful addressing uses fixed subnet masks (based on class A, B, or C), CIDR introduces the concept of VLSMs, allowing subnet masks of varying lengths.

- **Elimination of Address Classes**: CIDR does away with the rigid class A, B, and C networks, favoring a more fluid system where network boundaries are defined by the needs of the organization rather than predetermined class sizes.

- **Improved Utilization of Address Space**: Classful addressing often leads to unused addresses, particularly in large networks. CIDR's flexibility helps avoid this problem by tailoring subnet sizes to the actual number of hosts.

Overall, CIDR has greatly enhanced IP address efficiency, routing, and network scalability, providing a robust foundation for modern Internet infrastructure.

What does CIDR mean?

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