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Classic subnetting gives every subnet the same size. That's fine for a computer lab but awful for real networks, where a point-to-point WAN link needs 2 addresses and a user VLAN needs 500. VLSM — Variable Length Subnet Masking — lets you size each subnet to its need.
Say you subdivide `192.168.0.0/24` into four /26 subnets, each supporting 62 hosts. If three subnets need 60 hosts each and one is a WAN link needing 2, you just burned 60 addresses on the WAN link. Multiply by dozens of WAN links per site and you've wasted thousands of addresses.
VLSM flips the approach: allocate each subnet from the parent block with whatever prefix length it actually needs. Large subnets get short prefixes, small subnets get long prefixes, and everything fits in the same parent block.
Sorting largest-first matters because a small subnet placed early can fragment the remaining space so that a large subnet no longer fits contiguously. Do the big ones first and the small ones slot into the gaps.
Allocate from `192.168.1.0/24`. Requirements: VLAN 10 (100 hosts), VLAN 20 (50 hosts), VLAN 30 (20 hosts), WAN link (2 hosts).
| Requirement | Hosts needed | Prefix | Block | Range |
|---|---|---|---|---|
| VLAN 10 | 100 | /25 | 192.168.1.0/25 | .0 – .127 |
| VLAN 20 | 50 | /26 | 192.168.1.128/26 | .128 – .191 |
| VLAN 30 | 20 | /27 | 192.168.1.192/27 | .192 – .223 |
| WAN link | 2 | /30 | 192.168.1.224/30 | .224 – .227 |
| Unused | — | — | 192.168.1.228 – .255 | 28 addresses reserved |
Notice the /30 WAN link uses only 4 addresses but leaves 28 unused at the end. If you'd sized everything uniformly, you'd have wasted far more on the WAN link alone. VLSM's efficiency comes from matching size to need.
When allocating manually, always verify the next block starts exactly where the previous one ended. `192.168.1.128/26` ends at `.191`; the next block must start at `.192`, not `.200` or `.196`. A one-off miscount causes an overlap that's painful to debug — two subnets claiming the same address.
You're allocating from 10.10.0.0/22 (1024 addresses). Requirements: 200 hosts, 100 hosts, 50 hosts, 25 hosts, and 4 point-to-point WAN links needing 2 hosts each. What allocation order prevents overlap?
Select the safest next action.
VLSM in hand. Next lesson applies it to realistic network designs, where you'll handle multiple sites, growth headroom, and summarization.
Finish the lesson once you have worked through the material. This awards ★ 35 XP.