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📘 08-10: Variable Length Subnet Mask (VLSM)

Network Systems

Module 08: Subnets and VLANs

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❓ What is VLSM?

VLSM (Variable Length Subnet Mask) is a technique that allows a network to be divided into subnets of different sizes.

Unlike fixed subnetting, VLSM allows more efficient use of IP addresses by allocating subnets based on actual requirements.

💡 Why VLSM is Needed

In traditional subnetting: - all subnets are of equal size
- leads to wasted IP addresses

VLSM solves this by: - allocating larger subnets to networks with more hosts
- allocating smaller subnets to networks with fewer hosts

👉 This improves IP address utilization.

💡 Key Idea

Instead of dividing a network equally,

👉 VLSM divides a network based on need.

🧭 Example Scenario

Suppose we have a network:

  • Network: 192.168.1.0/24

We need to create subnets for:

Department Required Hosts
A 100
B 50
C 25
D 10

📌 Step 1: Sort Requirements (Descending Order)

Always allocate the largest subnet first.

Department Hosts Needed
A 100
B 50
C 25
D 10

🧮 Step 2: Determine Required Subnet Sizes

Use formula:

Total addresses needed = hosts + 2

Department Hosts Required Closest Power of 2 Comments Prefix
A 100 102 128 = 2^7 7 host bits → (32 − 7) = 25 network bits /25
B 50 52 64 = 2^6 6 host bits → (32 − 6) = 26 network bits /26
C 25 27 32 = 2^5 5 host bits → (32 − 5) = 27 network bits /27
D 10 12 16 = 2^4 4 host bits → (32 − 4) = 28 network bits /28

🧮 Step 3: Assign Subnets Sequentially

Start from 192.168.1.0 and allocate in order.

🧮 Subnet A (/25)

  • Network: 192.168.1.0/25
  • Range: 192.168.1.0 → 192.168.1.127
  • Hosts: 192.168.1.1 → 192.168.1.126

🧮 Subnet B (/26)

  • Network: 192.168.1.128/26
  • Range: 192.168.1.128 → 192.168.1.191
  • Hosts: 192.168.1.129 → 192.168.1.190

🧮 Subnet C (/27)

  • Network: 192.168.1.192/27
  • Range: 192.168.1.192 → 192.168.1.223
  • Hosts: 192.168.1.193 → 192.168.1.222

🧮 Subnet D (/28)

  • Network: 192.168.1.224/28
  • Range: 192.168.1.224 → 192.168.1.239
  • Hosts: 192.168.1.225 → 192.168.1.238

📌 Remaining Address Space

Unused range: - 192.168.1.240 → 192.168.1.255

👉 Can be used for future subnets

💡 Why Order Matters

If smaller subnets are assigned first: - large contiguous blocks may not remain
- subnetting may fail

👉 Always allocate from largest to smallest.

📌 Advantages of VLSM

  • Efficient use of IP addresses
  • Flexible network design
  • Supports hierarchical addressing
  • Reduces wastage

🧮 VLSM vs Fixed Subnetting

Feature Fixed Subnetting VLSM
Subnet size Equal Variable
Efficiency Low High
Flexibility Limited High
Address usage Wasteful Optimized

💡 Key Idea

VLSM allows subnet sizes to vary,

👉 making network design more efficient and scalable.