VLSM Network Design
Design efficient networks using Variable-Length Subnet Masking to minimize address waste.
What Is VLSM?
VLSM (Variable-Length Subnet Masking) allows you to divide a network into subnets of different sizes. Without VLSM, all subnets must use the same mask, wasting addresses in smaller segments.
The Problem VLSM Solves
Imagine you have 10.0.0.0/24 (254 hosts) and need:
- Department A: 100 hosts
- Department B: 50 hosts
- Server room: 10 hosts
- Point-to-point link: 2 hosts
Without VLSM, you'd use /25 for everything (126 hosts each) — but you'd need 4 subnets of 126, which exceeds your /24 allocation.
With VLSM, you assign different-sized subnets based on actual need.
VLSM Design Process
Step 1: Sort by Size (Largest First)
Always allocate the largest subnet first to maintain contiguous address space.
| Segment | Hosts Needed | Subnet Size |
|---|---|---|
| Dept A | 100 | /25 (126 hosts) |
| Dept B | 50 | /26 (62 hosts) |
| Servers | 10 | /28 (14 hosts) |
| P2P Link | 2 | /30 (2 hosts) |
Step 2: Allocate Sequentially
Starting from 10.0.0.0/24:
10.0.0.0/25 → Dept A (10.0.0.1 – 10.0.0.126) 126 hosts
10.0.0.128/26 → Dept B (10.0.0.129 – 10.0.0.190) 62 hosts
10.0.0.192/28 → Servers (10.0.0.193 – 10.0.0.206) 14 hosts
10.0.0.208/30 → P2P Link (10.0.0.209 – 10.0.0.210) 2 hosts
Step 3: Verify No Overlap
Each subnet's range must not overlap with any other: - /25 ends at .127 → /26 starts at .128 ✓ - /26 ends at .191 → /28 starts at .192 ✓ - /28 ends at .207 → /30 starts at .208 ✓
Address Utilization
| Method | Addresses Used | Addresses Wasted |
|---|---|---|
| Fixed /25 | 504 (4×126) | Exceeds /24! |
| Fixed /26 | 248 (4×62) | 86 wasted |
| VLSM | 204 | 50 remaining (available for future use) |
VLSM uses 80% of the address space efficiently, compared to fixed-length subnetting.
Key Rules
- Subnets must fall on power-of-2 boundaries
- Always allocate largest subnets first
- Verify no overlapping ranges
- Reserve space for future growth where possible
- Point-to-point links use /30 (or /31 per RFC 3021)