What is CIDR?
Classless Inter-Domain Routing isn't just jargon. It's the system that saved the internet from running out of addresses in the 90s.
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and IP routing. It replaced the rigid "classful" system (Class A, B, C) with a flexible notation that allows network engineers to slice IP ranges into smaller, more efficient subnets.
This efficiency is crucial for BGP routing between Autonomous Systems, keeping the global routing table manageable.
Reading CIDR Notation
CIDR notation consists of an IP address, a slash, and a number (the prefix length).
The /24 tells us that the first 24 bits of the address identify the network, leaving the remaining 8 bits for host devices (256 IPs).
Why It Matters
Efficiency
Before CIDR, organizations were assigned huge blocks of IPs they didn't need. CIDR allows assigning the exact number required.
Routing Table Size
CIDR allows "route aggregation" or supernetting. One routing entry can cover hundreds of smaller networks, keeping the global routing table manageable.
Frequently Asked Questions
How many usable IPs are in a subnet?
The formula is 2^(32-prefix) - 2. We subtract 2 because the very first IP is the Network Address and the very last is the Broadcast Address.
What is a Subnet Mask?
A subnet mask is just another way to write the prefix length. For a /24 network, the mask is 255.255.255.0.