📘 08-05: CIDR (Classless Inter-Domain Routing)¶
❓ What is CIDR?¶
CIDR (Classless Inter-Domain Routing) is a method of IP address allocation that allows flexible network boundaries.
It replaces classful networking by removing fixed classes (A, B, C) and allowing networks to be defined using prefix lengths.
💡 Why CIDR is Needed¶
Classful networking had several limitations:
- fixed network sizes (/8, /16, /24)
- inefficient use of IP addresses
- lack of flexibility
CIDR solves these issues by:
- allowing variable-length subnet masks
- enabling efficient allocation of IP addresses
📌 CIDR Notation¶
CIDR represents an IP address using a prefix length.
Format:
IP Address / Prefix Length
Example:
- 192.168.1.0/24
- 10.0.0.0/8
- 172.16.0.0/16
📌 Understanding Prefix Length¶
The prefix length indicates the number of bits used for the network portion.
- /24 → first 24 bits = network
- /16 → first 16 bits = network
- /8 → first 8 bits = network
The remaining bits are used for hosts.
🧮 CIDR and Subnet Mask¶
CIDR notation corresponds directly to a subnet mask.
| CIDR | Subnet Mask |
|---|---|
| /8 | 255.0.0.0 |
| /16 | 255.255.0.0 |
| /24 | 255.255.255.0 |
| /26 | 255.255.255.192 |
📌 Binary Representation¶
CIDR uses continuous 1s to represent the network portion.
Example: /26
11111111.11111111.11111111.11000000
- First 26 bits → Network
- Last 6 bits → Host
📌 CIDR vs Classful Networking¶
| Feature | Classful | CIDR |
|---|---|---|
| Network sizes | Fixed | Flexible |
| Efficiency | Low | High |
| Subnetting | Limited | Fully supported |
| Address usage | Wasteful | Optimized |
🧮 CIDR and Subnetting¶
CIDR is the foundation of subnetting.
Subnetting works by:
- increasing the prefix length
- borrowing bits from the host portion
Example:
- 192.168.1.0/24 → original network
- 192.168.1.0/26 → subnetted network
👉 Prefix increases → more subnets, fewer hosts per subnet
🧭 CIDR in Routing¶
CIDR is also used in routing to summarize networks.
Example:
Instead of multiple routes:
- 192.168.1.0/24
- 192.168.2.0/24
- 192.168.3.0/24
We can summarize as: - 192.168.0.0/22
👉 This reduces routing table size and improves efficiency.
💡 Key Idea¶
CIDR removes fixed class boundaries,
👉 allowing networks to be defined using flexible prefix lengths.
This makes modern networking:
- scalable
- efficient
- adaptable
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