Subnetting
🏛️ The Rise and Fall of Classful Addressing — A Legacy of the Internet
🧠 The Birth of Classful Addressing: Internet’s Early Blueprint
Before the digital world exploded with smartphones, cloud platforms, and smart fridges, the internet needed a way to manage its growing user base. Enter: Classful Addressing, a system born in the 1980s that would lay the foundation for early IP allocation.
It was simple. It was structured. But ultimately… it was flawed.
📦 Understanding the Classes
In Classful Addressing, IPv4 addresses were divided into 5 classes based on their first octet:
| Class | First Octet | Subnet Mask | Address Range | Max Hosts | Use Case |
|---|---|---|---|---|---|
| A | 1–126 | 255.0.0.0 (/8) | 1.0.0.0 – 126.255.255.255 | ~16 million | Large organizations |
| B | 128–191 | 255.255.0.0 (/16) | 128.0.0.0 – 191.255.255.255 | ~65,000 | Medium-sized networks |
| C | 192–223 | 255.255.255.0 (/24) | 192.0.0.0 – 223.255.255.255 | 254 | Small networks |
| D | 224–239 | N/A | 224.0.0.0 – 239.255.255.255 | N/A | Multicast |
| E | 240–255 | N/A | 240.0.0.0 – 255.255.255.255 | N/A | Research/Testing |
🔁 Note: The
127.x.x.xrange is reserved for loopback addresses.
❌ Why Classful Addressing Failed
As the internet grew, Classful Addressing became inefficient:
| Problem | Description |
|---|---|
| IP Address Waste | A company needing 300 IPs would get a Class B (~65,000 IPs). Huge waste. |
| Fixed Block Sizes | No flexibility — only /8, /16, /24 were available. |
| Routing Table Bloat | Each network needed its own route. Routing tables became massive. |
| Inefficient Allocation | Organizations held massive unused IP ranges. |
💡 Enter CIDR: The Classless Savior
CIDR (Classless Inter-Domain Routing) was introduced in 1993 (RFC 1519) to fix Classful’s problems:
✅ Key Features:
- Eliminated rigid classes (A, B, C)
- Uses prefix notation (
/13,/21,/30) - Supports subnetting and supernetting
- Enables route summarization
🆚 Classful vs Classless (CIDR)
| Feature | Classful | CIDR (Classless) |
|---|---|---|
| Address Division | Fixed (A, B, C) | Flexible |
| Subnet Mask | Default per class | Any length (e.g., /27) |
| IP Allocation | Wasteful | Efficient |
| Routing Tables | Large and slow | Summarized/Optimized |
| Used Today? | ❌ Deprecated | ✅ Industry Standard |
🧠 TL;DR
- Classful Addressing was simple but highly inefficient.
- CIDR brought the flexibility, scalability, and efficiency that the modern internet needs.
🔧 CIDR & Subnetting: A Complete Technical Deep-Dive
Category: Technical / CCNA / Network+ Prep
1️⃣ What is CIDR?
CIDR (Classless Inter-Domain Routing) is the modern method for IP address allocation and routing.
- Uses prefix notation like
/24,/27 - Efficient allocation (no more wasted IPs)
- Supports supernetting and subnetting
💡 Example:
192.168.10.0/26 – /26 means 26 bits are used for the network portion.
2️⃣ What is Subnetting?
Subnetting divides a larger network into smaller, manageable subnets.
🧩 Why Subnet?
- Logical segmentation
- Better performance and security
- Reduced broadcast traffic
- Optimized IP usage
3️⃣ Subnetting Methods
📏 3.1 Fixed-Length Subnet Masking (FLSM)
- All subnets are equal in size
- Same subnet mask used everywhere
- Simple, ideal for labs
📐 3.2 Variable-Length Subnet Masking (VLSM)
- Subnets have different sizes
- Efficient and scalable
- Used in real-world networks
4️⃣ Subnetting by Borrowing Bits
| Borrowed Bits | Subnets | Hosts/Subnet |
|---|---|---|
| 1 | 2 | 126 |
| 2 | 4 | 62 |
| 3 | 8 | 30 |
| 4 | 16 | 14 |
| 5 | 32 | 6 |
⚠️ Subtract 2 hosts per subnet (1 for network, 1 for broadcast)
5️⃣ Subnet Attributes Explained
| Attribute | Description |
|---|---|
| Network Address | All host bits = 0 (first IP, not usable) |
| Broadcast Address | All host bits = 1 (last IP, not usable) |
| First Usable | Network Address + 1 |
| Last Usable | Broadcast Address – 1 |
| Max Hosts | 2^h – 2 (h = host bits) |
🧪 Example: Subnet 192.168.10.0/26
- Subnet Mask: 255.255.255.192
- Total IPs: 64
- Network Address: 192.168.10.0
- Broadcast Address: 192.168.10.63
- First Usable: 192.168.10.1
- Last Usable: 192.168.10.62
- Max Hosts: 62
6️⃣ Special Subnet Cases
🔹 /30 — Point-to-Point Links
- 4 total IPs: 2 usable
- Used in router-to-router links
🔹 /31 — RFC 3021
- 2 total IPs: both usable
- No network/broadcast
- Efficient for P2P links (supported in Cisco IOS)
🔹 /32 — Host Route
- 1 IP address (single host)
- Used for:
- Loopbacks
- ACLs
- Route summarization
7️⃣ VLSM Subnetting: Step-by-Step
Given: 192.168.1.0/24
| Subnet | Required Hosts | CIDR | Assigned Block | Usable IPs |
|---|---|---|---|---|
| A | 50 | /26 | 192.168.1.0/26 | .1 – .62 |
| B | 20 | /27 | 192.168.1.64/27 | .65 – .94 |
| C | 10 | /28 | 192.168.1.96/28 | .97 – .110 |
🔁 Start with largest subnet and assign downwards.
8️⃣ Subnet Mask Summary Table
| CIDR | Subnet Mask | Hosts/Subnet |
|---|---|---|
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
| /31 | 255.255.255.254 | 2 (P2P) |
| /32 | 255.255.255.255 | 1 (Host) |
🧠 Final Thoughts
Whether you’re running an enterprise network or prepping for your CCNA exam, understanding CIDR and subnetting is foundational.
🏁 Classful addressing is gone — but its lessons helped shape the efficient, scalable networks we rely on today.
🙌 Connect With Me
