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Week 20%

Lesson 4 — VLSM Basics

Week 2Lesson 4Required27 min estimated0% progress

VLSM Basics

Carve one address block into right-sized subnets with variable-length masks, allocating largest first so nothing overlaps and nothing is wasted.

Lesson orientation

What you'll learn (4 objectives)~27 min: video → lesson → check → apply → lab prep

Learning objectives

  • Explain why fixed-length subnetting wastes addresses and VLSM conserves them
  • Size each subnet to its host count by rounding up to a power of two
  • Allocate subnets largest-first from a single block without overlap
  • Verify a plan by checking that each subnet starts on a valid boundary for its own size

Terms you will see

VLSMFixed-length subnettingHost requirementSubnet boundaryAlignmentOverlapPoint-to-point link

Time breakdown

  • Read the notes12 min
  • Subnetting Trainer — VLSM mode10 min
  • Worked allocation and prediction5 min

Why not one mask for everything?

Fixed-length subnetting forces every subnet in a design to the same size. That is simple, and it is wasteful the moment your segments differ. If you split a block into /24s but one segment holds 200 devices and another holds a two-router link, the small segment burns 252 addresses it will never use.

Variable-length subnet masking lets each subnet take the prefix that fits its own host count. One block stretches much further, and it becomes practical to give a point-to-point link the /30 it actually needs rather than a whole /24. The arithmetic is nothing new — it is the block-size method from Lesson 2, applied repeatedly and in the right order.

Sizing a subnet from its host requirement

  1. Take the number of hosts the segment needs, including realistic growth.
  2. Find the smallest number of host bits h where 2^h − 2 is at least that requirement. The two lost addresses are always the network and broadcast.
  3. The block size is 2^h, and the prefix is 32 − h.
Host requirement to prefix — the table to be able to rebuild, not memorise
Hosts neededHost bitsBlock sizePrefixUsable hosts
1 – 224/302
3 – 638/296
7 – 14416/2814
15 – 30532/2730
31 – 62664/2662
63 – 1267128/25126
127 – 2548256/24254
Round up, and remember the two lost addresses
A segment with 62 hosts fits a /26 exactly, because 2^6 − 2 = 62. A segment with 63 hosts does not — it needs a /25. Sizing on 2^h rather than 2^h − 2 is the most common way to produce a plan that is one address short on its busiest segment.

Allocate largest first

Sort the requirements from largest to smallest, place the biggest subnet at the start of the block, and lay each subsequent subnet immediately after the previous one's broadcast address. Worked through on 192.168.1.0/24 for Engineering (100 hosts), Sales (50), Ops (20), and a router-to-router link (2):

192.168.1.0/24 allocated largest-first
SegmentHostsPrefixNetworkFirst hostLast hostBroadcast
Engineering100/25192.168.1.0192.168.1.1192.168.1.126192.168.1.127
Sales50/26192.168.1.128192.168.1.129192.168.1.190192.168.1.191
Ops20/27192.168.1.192192.168.1.193192.168.1.222192.168.1.223
R2R link2/30192.168.1.224192.168.1.225192.168.1.226192.168.1.227

Every subnet begins exactly where the previous broadcast plus one falls, and every start is a valid boundary for its own size: 128 is a multiple of 64, 192 is a multiple of 32, and 224 is a multiple of 4. Addresses from 192.168.1.228 onward are still free. That alignment is not luck — it is what descending order buys you, because a large block always lands on a boundary that the smaller blocks after it can also satisfy.

The two ways a plan goes wrong

These are different failures with different symptoms, and it is worth keeping them apart.

Failure 1 — overlap, from an unaligned start
A subnet must begin on a multiple of its own block size. Put a /26 at 192.168.1.32 and its range runs .32 to .95, straddling the .64 boundary and overlapping the /26 that belongs at .64. Two subnets then claim the same addresses, and hosts in the overlap behave unpredictably depending on which mask their own interface carries. Check every network address against its block size before you commit a plan.
Failure 2 — fragmentation, from allocating small first
Take the same four requirements but place the /30 link at 192.168.1.0 first. Engineering's /25 needs 128 aligned addresses, and .0 through .127 no longer has them, so Engineering is pushed to .128 through .255. Sales and Ops then fit into .64 and .32, but .4 through .31 — 28 addresses — is stranded in pieces too small for anything. The plan still works here, but it has wasted a tenth of the block, and on a tighter block the same fragmentation makes an otherwise feasible plan impossible.

So: allocate largest first to avoid wasting the block, and check alignment to avoid overlapping it. The exam tests both, and the two mistakes look nothing alike once you know what you are looking at.

Verifying a finished plan

  • Every network address is a multiple of its own block size.
  • Every subnet's broadcast is exactly one below the next subnet's network address, with no gap you did not intend.
  • No subnet's range extends into the next subnet's range.
  • Every segment's usable host count is at least its stated requirement, computed as 2^h − 2.
  • The total addresses allocated do not exceed the parent block.

Run those five checks on any plan before you configure anything. Four of them are arithmetic you can do in your head after this week, and they catch every error described above.

What you should retain

  • VLSM matches each subnet's prefix to its own host count, so one block stretches much further.
  • Size by rounding up to the smallest h where 2^h − 2 meets the requirement.
  • Allocate largest first, placing each subnet at the previous broadcast plus one.
  • A subnet must start on a multiple of its own block size — unaligned starts cause overlap.
  • Small-first allocation does not usually overlap; it fragments the block and strands the large subnets.
  • Point-to-point links take a /30 and give exactly 2 usable addresses.
Pause and predictNot scored — nothing is recorded

Before you read on

Continue the allocation above. A second router-to-router link needs 2 hosts, and the next free address is 192.168.1.228. Give its prefix, network address, usable range, and broadcast — and confirm the start is properly aligned.

Interactive tool

See it happen

Required~10 min

Subnetting Trainer — VLSM mode

Switch the trainer into VLSM mode and allocate a few blocks end to end. Doing several by hand is what turns the largest-first rule from something you agree with into something you actually do under time pressure.

Open the tool
Section quiz

Check this section before moving on

Required8 questions~10 min

Wildcards & VLSM Allocation

The quiz opens on its own page so you can focus on it. It is a Week 1 milestone, tracked separately from this lesson's own completion.

Topics covered

  • • Subnet mask to wildcard conversion and what a wildcard matches
  • • Sizing a subnet from its host requirement
  • • Largest-first allocation and subnet boundaries
  • • Spotting overlap and fragmentation in a plan
Start the quiz

Study deeper

Topic guides extend this lesson — they do not replace the first-party walkthrough above.

IP Addressing & Subnetting

For additional VLSM worked examples and the wider addressing context behind this week's arithmetic

Open guide