Variable Length Subnetting (VLSM)

VLSM stands for Variable-Length Subnet Masking, which allows subnetting a network into subnets of different sizes based on need. Instead of using the same subnet mask for all subnets (as in fixed-length subnetting), VLSM enables more efficient use of IP addresses.

Why Use VLSM

In real-world networks:

• Not all departments or locations need the same number of IP addresses.
• Using fixed-length subnets leads to wasted IPs.
• VLSM solves this by assigning larger subnets to bigger networks and smaller subnets to smaller networks.

Key Concepts in VLSM

• Start with the largest subnet requirement first.
• Borrow bits from the host portion as needed.
• Always arrange subnets from largest to smallest to avoid overlaps.
• VLSM is supported in classless routing protocols (e.g., OSPF, EIGRP, BGP), but not in RIP v1.

Example of VLSM

You are given a network: 192.168.1.0/24
You need to divide it into subnets for:

Step-by-Step VLSM

Step 1: Sort by host needs (largest to smallest)

• A – 100 hosts → Needs at least /25 (supports 126 usable IPs)
• B – 50 hosts → Needs /26 (62 usable)
• C – 20 hosts → Needs /27 (30 usable)
• D – 10 hosts → Needs /28 (14 usable)

Step 2: Assign subnets

• 192.168.1.0/25 → Department A (IP range: 192.168.1.1 to 192.168.1.126)
• 192.168.1.128/26 → Department B (192.168.1.129 to 192.168.1.190)
• 192.168.1.192/27 → Department C (192.168.1.193 to 192..222)
• 192.168.1.224/28 → Department D (192.168.1.225 to 192.168.1.238)

You’ve efficiently used IPs with no waste.

Benefits of VLSM

Here are some key benefits of VLSM

• Efficient IP address utilization
• Flexible subnetting
• Reduces IP wastage
• Ideal for real-world, hierarchical network design