• VLSM allows different subnet masks within the same major network to maximize IP address efficiency
• Enables subnetting a subnet (hierarchical subnetting) by using different prefix lengths for different network segments
• Must use classless routing protocols (RIPv2, EIGRP, OSPF, BGP) that advertise subnet mask information
• Solves the waste problem of fixed-length subnet masking where all subnets must use the same mask regardless of host requirements

How VLSM Works

• Start with largest subnet requirement first, then work down to smallest
• Each subsequent subnet uses remaining address space from the original network
• Subnet masks can vary from /8 to /30 (point-to-point links) within same major network
• Key principle: Allocate exactly what you need, no more, no less

VLSM Design Process

1. List requirements - Identify all subnets and their host counts
2. Sort by size - Largest subnet first (prevents overlap)
3. Assign addresses - Use next available block that fits requirement
4. Calculate ranges - Document network, broadcast, and usable host ranges
5. Verify no overlap - Ensure subnet ranges don’t conflict

Practical Example

Starting with 192.168.1.0/24 network:

• Sales (50 hosts needed): 192.168.1.0/26 (62 usable hosts)
• Engineering (25 hosts): 192.168.1.64/27 (30 usable hosts)
• Marketing (10 hosts): 192.168.1.96/28 (14 usable hosts)
• WAN links (2 hosts each): 192.168.1.112/30, 192.168.1.116/30

VLSM vs Fixed Length Subnetting

Vocabulary

• Classless routing protocol: Routing protocol that includes subnet mask in updates (RIPv2, EIGRP, OSPF)
• Hierarchical subnetting: Process of subnetting an already subnetted network
• Address aggregation: Combining multiple subnets into single routing advertisement (summarization benefit)
• Subnet overlap: Configuration error where subnet ranges conflict

Notes

• Always start with largest subnet first - prevents running out of contiguous address space
• VLSM requires careful planning - draw out your addressing scheme before implementation
• Use /30 masks for point-to-point WAN links (only 2 usable addresses needed)
• Leave room for growth - don’t use every available address immediately
• Routing table entries increase with VLSM but can be reduced through route summarization
• Common mistake: Forgetting that classful routing protocols (RIPv1, IGRP) cannot support VLSM
• Best practice: Document your VLSM scheme thoroughly - complex addressing requires good records
• VLSM enables route summarization at network boundaries, reducing routing table size in large networks