How does CIDR work?

CIDR (Classless Inter-Domain Routing) works by using a flexible method for allocating IP addresses and managing routing. Here’s how it functions:

1. IP Address Structure: An IP address consists of two parts: the network part and the host part. CIDR allows for variable-length subnet masking, meaning the division between these two parts can be adjusted based on the needs of the network.

2. Subnet Mask: In CIDR notation, the subnet mask is represented by a slash (/) followed by a number that indicates how many bits are used for the network part. For example, in 192.168.1.0/24, the /24 means that the first 24 bits are the network part, and the remaining 8 bits are for host addresses.

3. Route Aggregation: CIDR enables the aggregation of multiple IP addresses into a single routing entry. This is done by summarizing a range of IP addresses into a single CIDR block. For example, the addresses 192.168.0.0, 192.168.1.0, and 192.168.2.0 can be aggregated into 192.168.0.0/22.

4. Routing Decisions: Routers use CIDR to make routing decisions based on the most specific match. When a packet is sent, the router checks its routing table for the longest prefix match (the most specific CIDR notation) to determine where to forward the packet.

5. Dynamic Allocation: CIDR allows for dynamic allocation of IP addresses, which means that IP addresses can be assigned based on current needs rather than fixed classes. This flexibility helps in optimizing the use of available IP address space.

By using CIDR, networks can be designed more efficiently, leading to better utilization of IP addresses and improved routing performance.