VLSM Subnet Calculator
Enter a base network and your host requirements. Use one line per subnet in the format Name,Hosts or Name:Hosts.
What is subnetting with VLSM?
VLSM (Variable Length Subnet Mask) is a subnetting method that lets you assign different subnet sizes based on real host needs. Instead of splitting a network into equal-size chunks, VLSM gives larger teams larger subnets and smaller links smaller subnets. This saves IP space and makes network designs much cleaner.
Why use a VLSM calculator?
Manual subnetting is a great skill, but in production environments speed and accuracy matter. A calculator helps you:
- Allocate subnets from largest to smallest automatically.
- Avoid wasted addresses and avoid overlap mistakes.
- See network ID, usable host range, broadcast address, and mask instantly.
- Validate whether your base network is large enough for all requested segments.
How this calculator works
1) Parse the base network
The tool takes your CIDR block (for example, 10.20.0.0/22) and computes the total address pool.
2) Parse host requirements
Each line is treated as a subnet request. If you enter Branch-A,120, the calculator reserves a block that can support at least 120 usable hosts.
3) Allocate from largest to smallest
To preserve alignment and reduce fragmentation, requests are sorted by host size before assignment. This is the standard VLSM best practice.
4) Output practical addressing details
You get the subnet CIDR, dotted mask, network address, usable range, broadcast address, and usable host capacity for each line item.
Quick VLSM design checklist
- Document all segments (users, servers, IoT, WAN links, management).
- Estimate growth; don’t subnet only for today.
- Sort by largest host requirement first.
- Reserve spare space for future VLANs or site expansion.
- Keep WAN and point-to-point links in compact subnets.
Example scenario
Suppose your base block is 192.168.10.0/24 and your needs are:
- Sales: 60 hosts
- Engineering: 28 hosts
- HR: 12 hosts
- WAN: 2 hosts
A valid VLSM allocation typically becomes /26, /27, /28, and /30 subnets in descending order. That allocation uses address space efficiently and leaves room for additional small segments.
Common subnetting mistakes to avoid
- Allocating in random order: always place the largest subnet first.
- Forgetting network and broadcast: usable hosts are less than total block size.
- Misaligned boundaries: subnet starts must align with block size.
- No growth planning: leave headroom for expansion.
Final thoughts
VLSM is essential for modern IPv4 network design. A good calculator helps engineers move quickly while staying accurate. Use this page for planning VLANs, branch office layouts, lab topologies, and certification practice.