Visual Lab
Lv 1
Visual lab

Learn subnetting visually

11 short lessons, from binary to IPv6. Change the inputs and watch every value update.

Lesson 8 of 11Advanced

Design with VLSM

What is VLSM?
Variable Length Subnet Masking gives each subnet only the size it needs instead of cutting the network into equal pieces. A 2-host WAN link gets a /30 while a 60-host LAN gets a /26, all from one 192.168.10.0/24.

The requirements

Edit the host counts. The plan below updates; the network stays 192.168.10.0/24.

Build the plan

Step through it, or jump to the end.

.0.64.128.192.255
1

Sort largest first

  1. Eng60
  2. Sales25
  3. Mgmt12
  4. WAN2
Step 1 of 7

Why largest first?

Here is the same list packed smallest first, each block right after the last.

  • WAN needs /30 (block 4)192.168.10.0/30 aligned
  • Mgmt needs /28 (block 16)192.168.10.4/28 not a multiple of 16
  • Sales needs /27 (block 32)192.168.10.20/27 not a multiple of 32
  • Eng needs /26 (block 64)192.168.10.52/26 not a multiple of 64

Red rows are not valid subnets: a network address must have every host bit set to 0. Fixing them means skipping ahead to the next boundary, which leaves gaps. Placing the largest block first keeps every later, smaller block on a boundary automatically.

Lock it in

Drill the “VLSM design” skill until it is automatic, or try the real tool.

Lesson 8 of 11
Next: Summaries and route lookup