VLSM Subnet Calculator (Variable Length Subnet Masking)

Space Allocation: 124 / 256 IPs (48.4%)Within Network Limits

Required Subnet Allocations (5)

1
Hosts:
2
Hosts:
3
Hosts:
4
Hosts:
5
Hosts:

Calculated VLSM Allocation Table

Subnet NameNeeded HostsAllocatedNetwork AddressNetmask / CIDRUsable Host RangeBroadcast IPWildcard Mask
HQ Engineering6062192.168.10.0255.255.255.192 /26192.168.10.1 - 192.168.10.62192.168.10.630.0.0.63
Sales & Marketing2830192.168.10.64255.255.255.224 /27192.168.10.65 - 192.168.10.94192.168.10.950.0.0.31
Customer Support1414192.168.10.96255.255.255.240 /28192.168.10.97 - 192.168.10.110192.168.10.1110.0.0.15
IoT Device Lab66192.168.10.112255.255.255.248 /29192.168.10.113 - 192.168.10.118192.168.10.1190.0.0.7
Router P2P Link22192.168.10.120255.255.255.252 /30192.168.10.121 - 192.168.10.122192.168.10.1230.0.0.3
Major Block Capacity256 IPs
Total Needed Hosts110 Hosts
Total Allocated Hosts124 IPs
Address Space Used48.4%

100% Client-Side Topology

Your confidential enterprise network architecture and internal IP addressing plans are computed entirely in browser memory. Zero cloud telemetry.

Hierarchical Route Optimization

Sorts departments descending by required host count to prevent routing black holes, fragmented address space, and mask overlaps.

One-Click Markdown Export

Export generated allocation tables directly into GitHub Markdown, Notion, or network engineering documentation specifications.

Technical Reference & Architecture Guide

VLSM Subnet Calculation & Hierarchy Design Guide — Reference Guide

Comprehensive reference for Variable Length Subnet Masking (VLSM), route aggregation, binary prefix boundaries, and hierarchical IP space planning.

Key Parameters & Controls

majorNetworkstring (e.g. 192.168.1.0/24)

The base IPv4 CIDR address block available for allocation.

neededHostsnumber

The number of usable host IP addresses required for each specific subnet.

allocatedHostsnumber (2^n - 2)

Actual usable hosts provided by the power-of-two subnet block.

networkAddressIPv4 address

First IP of the block, representing the subnet wire address.

broadcastAddressIPv4 address

Last IP of the block, used for all-hosts broadcast traffic.

Pro Tips & Best Practices

Always sort subnet host requirements in descending order before computing VLSM to prevent routing holes and fragmented address space.

Account for 2 reserved addresses in every subnet: the network wire ID (.0) and the broadcast address (.255).

Point-to-point router serial links only require 2 usable hosts, making /30 (255.255.255.252) or RFC 3021 /31 optimal choices.

Common Gotchas & Troubleshooting

Expand your major network mask (e.g. from /24 to /23 or /22) or reduce individual subnet host allocations.

Ensure strict power-of-two boundaries are respected and subnets are allocated sequentially from largest to smallest.

Frequently Asked Questions

What is the primary difference between FLSM and VLSM?▼

Fixed Length Subnet Masking (FLSM) forces every subnet to the same size, wasting address space. VLSM tailors each subnet size dynamically to actual host demand.

How does the tool calculate the required CIDR prefix for a given host count?▼

It finds the smallest integer n such that 2^n - 2 >= neededHosts, and assigns a prefix of 32 - n.

Are my network topology plans kept private?▼

Yes. 100% of the binary IP calculations, bit-shifts, and table exports occur in your client browser memory.

Can I export the calculated VLSM table into my documentation?▼

Yes, you can copy the entire subnet breakdown table directly as formatted Markdown with one click.

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