Full form: Classless Inter-Domain Routing
Also known as: Classless Inter-Domain Routing, prefix notation, slash notation
Quick Definition
A method of IP address allocation that uses prefix notation instead of fixed class boundaries.
CIDR replaces the old classful system (Class A, B, C) with variable-length prefix notation. An IP address is written with a slash followed by the number of network bits (e.g., 10.0.0.0/22). This allows subnets of any size, reduces IP address waste, and enables route summarisation. CIDR is the foundation of modern IP addressing and subnetting.
10.0.0.0/22 is a CIDR block containing 1024 addresses (4 × /24 subnets). A router can summarise 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24, and 10.0.3.0/24 into a single /22 advertisement.
The CCNA exam heavily tests subnetting with CIDR notation. Make sure you can quickly determine the number of hosts (/prefix host count), the network address, broadcast address, and usable host range.
A 32-bit value that divides an IP address into the network and host portions.
Using different subnet mask lengths within the same major network to minimise IP waste.
Using the same subnet mask length for all subnets in a network.
The inverse of a subnet mask, used in ACLs and OSPF network statements.
CIDR replaces the old classful system (Class A, B, C) with variable-length prefix notation. An IP address is written with a slash followed by the number of network bits (e.g., 10.0.0.0/22). This allows subnets of any size, reduces IP address waste, and enables route summarisation. CIDR is the foundation of modern IP addressing and subnetting.
The CCNA exam heavily tests subnetting with CIDR notation. Make sure you can quickly determine the number of hosts (/prefix host count), the network address, broadcast address, and usable host range.
10.0.0.0/22 is a CIDR block containing 1024 addresses (4 × /24 subnets). A router can summarise 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24, and 10.0.3.0/24 into a single /22 advertisement.
CIDR falls under the IP Addressing domain of the 200-301 exam. Understanding it in context with related terms like subnet-mask and vlsm is essential for answering scenario-based questions correctly.