What is VLSM and why is it more efficient than fixed-length subnetting?

VLSM lets you use different mask lengths within one network ("subnet a subnet"), so point-to-point links get tiny /30s and big LANs get large blocks — far less waste than giving every subnet the same fixed size.

VLSM (Variable Length Subnet Mask):

Allows using different subnet masks within the same network to allocate addresses efficiently.

Problem with Fixed-Length Subnetting:

• Every subnet gets the same size
• Small subnets waste addresses
• Example: Using /24 for a WAN link with 2 devices wastes 252 addresses

VLSM Solution:

• Use larger subnets where needed (more hosts)
• Use smaller subnets for point-to-point links (/30 or /31)

VLSM Design Process:

1. Start with the largest subnet requirement
2. Allocate subnets from largest to smallest
3. Use /30 for point-to-point links (2 hosts needed)

Example:

Key insight: VLSM requires routing protocols that support it (OSPF, EIGRP, RIPv2) - NOT RIPv1.

Go deeper:

• Variable Length Subnet Mask explained (LearnCisco) — a CCNA-oriented VLSM walk-through with a block-size/host table.
• Classless Inter-Domain Routing (Wikipedia) — the classless addressing that makes per-subnet masks possible.