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Networking Fundamentals · Foundational

Classless Inter-Domain Routing (CIDR)

The addressing scheme (RFC 4632) that replaced classful addressing by allowing any prefix length and route aggregation.

Definition

Introduced in 1993, CIDR lets address blocks be allocated in any size, using slash notation to show the prefix. It slowed IPv4 exhaustion and shrank Internet routing tables through aggregation. CIDR notation is now standard for both IPv4 and IPv6.

Example

An ISP allocating a /22 (1024 addresses) to a customer is using CIDR.

Exam tip

"CIDR notation" just means slash notation, such as 192.168.0.0/22.

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