🎯 Guiding Question
How do CIDR and VLSM improve network efficiency and IP address management in modern networks?
📘 Learning Objectives
- Explain the purpose of subnetting
- Describe how subnetting reduces network congestion and improves security
- Differentiate between classful and classless addressing
- Use CIDR notation to calculate subnet sizes and available hosts
- Explain how VLSM allows different-sized subnets
🌐 IPv4 Addresses
- An IPv4 address is a 32-bit number
- It identifies a device (host) on a network
- Every IP address includes a subnet mask
🧩 Subnet Mask
- A subnet mask is a 32-bit number
- It separates the network portion from the host portion
- Devices use it to determine local vs. remote destinations
Example
IP Address: 192.168.12.204
Decimal Subnet Mask: 255.255.255.0
Binary Subnet Mask:
11111111.11111111.11111111.00000000
🏷️ Classful Addressing
| Class | 1st Octet Range | Default Subnet Mask | Hosts |
|---|---|---|---|
| A | 1 – 126 | 255.0.0.0 | 16,777,214 |
| B | 128 – 191 | 255.255.0.0 | 65,534 |
| C | 192 – 223 | 255.255.255.0 | 254 |
❌ Problem: Class A and B are too large, Class C is often too small.
✅ Classless Addressing (CIDR)
- Subnet masks do not need to end on octet boundaries
- The first octet no longer determines the subnet
- Default subnet masks are not used
- Networks are sized based on actual needs
CIDR = Classless Inter-Domain Routing
Used on the Internet and modern internal networks.
Used on the Internet and modern internal networks.
✍️ CIDR Notation
CIDR notation shows the number of network bits.
192.168.50.0 /27
- /27 = 27 bits set to 1
- Decimal subnet mask: 255.255.255.224
📝 CIDR Practice (Solved)
| IP Address | CIDR | Decimal Subnet Mask |
|---|---|---|
| 177.100.18.4 | /18 | 255.255.192.0 |
| 10.10.250.1 | /13 | 255.248.0.0 |
| 193.100.77.83 | /28 | 255.255.255.240 |
| 95.250.91.99 | /22 | 255.255.252.0 |
| 189.210.50.1 | /17 | 255.255.128.0 |
| 220.90.130.45 | /27 | 255.255.255.224 |
🔀 VLSM – Variable Length Subnet Masking
VLSM allows different subnet sizes within the same network.
| Department | Hosts | CIDR | Address Range |
|---|---|---|---|
| IT | 50 | /26 | 192.168.1.0 – 192.168.1.63 |
| HR | 20 | /27 | 192.168.1.64 – 192.168.1.95 |
| Finance | 10 | /28 | 192.168.1.96 – 192.168.1.111 |
📎 Appendix: Binary System Review
- Binary is base 2
- Digits: 0 and 1
- One byte = 8 bits
128 64 32 16 8 4 2 1
Binary → Decimal Practice
01001101 = _____
10000110 = _____