Lesson 37: Classless Inter-Domain Routing (CIDR)
In the early days of the internet, the rigid Class A/B/C system led to massive IP address waste. CIDR (Classless Inter-Domain Routing), introduced in the early 1990s, solved this problem and is the standard addressing method today.
CIDR Notation (The Slash Notation)
CIDR eliminates the concept of 'default' classes. Instead of relying on the first octet, CIDR explicitly specifies the length of the Network ID using a slash followed by a number (known as the prefix length).
- The prefix length indicates how many bits (from the left) are dedicated to the Network ID.
- This slash notation is interchangeable with the subnet mask.
Example Conversions:
| CIDR Notation | Subnet Mask (Decimal) | Network Bits |
|---|---|---|
| /8 | 255.0.0.0 | 8 |
| /16 | 255.255.0.0 | 16 |
| /24 | 255.255.255.0 | 24 |
| /27 | 255.255.255.224 | 27 |
Advantages of CIDR
- IP Conservation: Allows for flexible subnetting (VLSM - Variable Length Subnet Masking), meaning you can create subnets exactly the size you need, instead of wasting thousands of addresses.
- Routing Efficiency: Enables route summarization (or supernetting), where a single route entry can represent multiple smaller networks, drastically reducing the size of routing tables on the internet.
Modern Standard: When you see an address like
10.1.1.0/24, you are using CIDR notation, which immediately tells you the first 24 bits are the network portion.