VLSM Planner
One address block, several networks of different sizes. Enter the base block and each subnet's host requirement — the planner allocates largest-first (the standard VLSM method) and shows the full breakdown you'd otherwise work out octet by octet.
| Subnet | Hosts | Network | Mask | Host range | Broadcast |
|---|---|---|---|---|---|
| Sales LAN | 60 → 62 | 192.168.10.0/26 | 255.255.255.192 | 192.168.10.1 – 192.168.10.62 | 192.168.10.63 |
| Engineering LAN | 28 → 30 | 192.168.10.64/27 | 255.255.255.224 | 192.168.10.65 – 192.168.10.94 | 192.168.10.95 |
| Servers | 12 → 14 | 192.168.10.96/28 | 255.255.255.240 | 192.168.10.97 – 192.168.10.110 | 192.168.10.111 |
| Router link | 2 → 2 | 192.168.10.112/30 | 255.255.255.252 | 192.168.10.113 – 192.168.10.114 | 192.168.10.115 |
116 of 256 addresses allocated · next free address: 192.168.10.116
A VLSM plan on paper is a hypothesis. In the NetForge-AI lab you can wire the routers, assign each carved subnet to an interface, and prove every PC reaches every server — or see precisely which link you mis-addressed.
Open the free routing lessonThe method, by hand
Take the classic case: you're given 192.168.10.0/24 and need subnets for 60, 28, and 12 hosts plus a router-to-router link. Sort descending, then for each: find the smallest prefix whose usable count (2host bits − 2) fits.
60 hosts → /26 (62 usable) at 192.168.10.0. 28 hosts → /27 (30 usable) at .64. 12 hosts → /28 (14 usable) at .96. The link → /30 (2 usable) at .112. Half the /24 is still free for growth — that's the entire point of VLSM over fixed-size subnetting.
The worked version of exactly this problem — with the packet animations showing why a wrong mask breaks reachability — is in Module 2 · IP Addressing & Subnetting.
VLSM FAQ
What is VLSM?
Variable Length Subnet Masking: instead of splitting a network into equal-size subnets, each subnet gets a prefix sized to its actual host count. A 60-host LAN gets a /26, a 12-host server segment a /28, and a router-to-router link a /30 — no addresses wasted on links that will only ever hold two devices.
Why must VLSM allocate largest subnets first?
Every subnet must start on a multiple of its own size. If you allocate a small subnet first, the next large one has to skip ahead to its alignment boundary, leaving unusable gaps. Sorting requirements largest-first keeps every allocation naturally aligned and gap-free.
How do I size a subnet for N hosts?
Find the smallest power of two that is at least N + 2 (network + broadcast). 60 hosts needs 64 addresses → 6 host bits → /26. 12 hosts needs 16 → /28. A point-to-point link needs 4 → /30 (or /31 where both ends support RFC 3021).
Does this match how exams grade VLSM questions?
Yes — the planner uses the standard textbook method: sort by size descending, allocate sequentially from the base network, and size each subnet with the 2^n − 2 rule. The intermediate steps it shows are the ones you'd write by hand.
More free tools: Subnet Calculator · Wildcard Mask Converter