VLSM Basics

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Plain-English explanation

VLSM (Variable Length Subnet Masking) lets you subnet a subnet. Instead of one mask everywhere, you carve right-sized blocks from a parent network — largest requirements first, then the next largest from remaining space.

Example parent: 192.168.100.0/24

Need 50 hosts → /26 (62 usable) → use 192.168.100.0/26
Need 25 hosts → /27 (30 usable) → use 192.168.100.64/27
Need 5 hosts → /29 (6 usable) → use 192.168.100.96/29
Remaining space available for future allocations from .104 onward

Deep dive

Why largest first? A small subnet leaves awkward fragments; a large requirement may not fit a leftover gap if you allocated small blocks first.

Overlap check: Each new subnet must start at or after the previous subnet's broadcast + 1.

Summarization link: VLSM designs that align on bit boundaries allow cleaner route summaries upstream (Week 4–5 preview).

| Requirement | Min prefix | Usable | |-------------|------------|--------| | 100 hosts | /25 | 126 | | 50 hosts | /26 | 62 | | 25 hosts | /27 | 30 | | 10 hosts | /28 | 14 | | 2 hosts (WAN) | /30 | 2 |

Step-by-step — full VLSM table

Parent: 10.10.0.0/16. Requirements: 500 hosts, 200 hosts, 50 hosts, 2× WAN (/30).

500 → /23 (510 usable) → 10.10.0.0/23 (uses 0.0–1.255)
200 → /24 (254 usable) → 10.10.2.0/24
50 → /26 (62 usable) → 10.10.3.0/26
WAN1 → 10.10.3.64/30
WAN2 → 10.10.3.68/30

Document network, mask, range, and next available after each row.

Commands to know

Document and apply VLSM on Cisco

interface GigabitEthernet0/0 description LAN-500hosts ip address 10.10.0.1 255.255.254.0 ! interface GigabitEthernet0/1 description LAN-200hosts ip address 10.10.2.1 255.255.255.0

Troubleshooting

| Design flaw | Symptom | |-------------|---------| | Overlapping subnets | Random failures, ARP weirdness | | Too-small block | Cannot add hosts without renumbering | | Small-first allocation | Large requirement doesn't fit | | Ignoring broadcast boundary | Off-by-one overlap on exams |

Always draw a number line of address space when learning — pros still sketch for complex designs.

Exam relevance

Week 2 gate

Pass the subnetting trainer at 80%+ with 10 timed questions before moving to switching in Week 3.

Exam trap

"Which subnets overlap?" — list every allocated range including broadcast before picking an answer. One hidden overlap fails the whole design.

Practice checklist

Complete three full VLSM problems with a written table
Verify no overlap by checking start/end of each block
Given a flawed design, identify which two subnets collide
Allocate from 172.16.0.0/22 for four department sizes in one pass
Pass Week 2 subnetting gate before starting Week 3

What is the first step in a typical VLSM problem?

After assigning 192.168.100.0/26 from a /24, where does the next subnet typically start?

Learning objectives