VLSM lets you use different subnet masks inside the same network, so IP space is utilized efficiently.
Facts / key topics
- RFC: not a standalone RFC – part of the CIDR concept
- Layer 3
- Requires classless routing protocols (OSPF, EIGRP, RIPv2, IS-IS, BGP)
- Does not work with classful routing (RIPv1, IGRP)
- Used in: enterprise LAN/WAN, campus, ISP
- CCNA: calculation & understanding · CCNP: design, troubleshooting, summarization side-effects
Why does it exist?
Without VLSM every subnet must have the same size → wasted addresses and poor scalability. Typical needs: segments with different host counts, point-to-point links (/30, /31), loopbacks (/32).
How it works, step by step
- Identify host requirements per subnet
- Sort largest → smallest
- Choose suitable prefix (/26, /27, /30 …)
- Assign subnets sequentially – the largest subnet is designed first, smallest placed last
Key terminology
| Concept | Explanation |
|---|---|
| CIDR | Classless Inter-Domain Routing |
| Prefix length | /24, /27 etc. |
| Subnet boundary | Jump between subnet blocks |
| Longest Match | The most specific route wins |
| Element | CCNA | CCNP |
|---|---|---|
| VLSM | ✔ | ✔ |
| Route summarization | ✔ | ✔ |
| Discontiguous networks | ✖ | ✔ |
Dependencies & considerations
- The routing protocol must be classless
- Errors in subnet calculation → routing failures
- Summarization can hide specific routes (CCNP!)
Exam tips ⚠️
- See different subnet masks → it’s VLSM
- Always start with the largest subnet
- /30 = 2 usable hosts (the classic)