Plan Different Subnet Sizes in One Parent
Variable Length Subnet Masking (VLSM) lets each subnet use a prefix suited to its capacity requirement. Choose a parent network, add IPv4 host counts or explicit prefixes, and select Preview Allocation. The planner places the largest requirements first in the lowest available aligned space. Review the preview before applying it.
Worked Example: a Branch Office with 25% Growth
Start with 10.20.0.0/22 using the Standard profile. Add Staff with 100 hosts, Voice with 40, and Guest Wi-Fi with 60. Set Growth Allowance to 25%. With no fixed or reserved blocks, the result is:
| Network | Requested | With Growth | Allocation | Usable |
|---|---|---|---|---|
| Staff | 100 | 125 | 10.20.0.0/25 | 126 |
| Voice | 40 | 50 | 10.20.1.0/26 | 62 |
| Guest Wi-Fi | 60 | 75 | 10.20.0.128/25 | 126 |
Guest Wi-Fi needs a /25 after growth, so it is placed before the smaller Voice network. The allocations occupy 320 of the parent’s 1,024 addresses, leaving 704 unallocated. Include gateway addresses in your host requirements. Growth rounds fractional host counts upward; explicit prefix requests do not change.
Keep Existing Networks & Reserve Future Space
Add an existing CIDR as a fixed allocation or mark a block as reserved. Both are excluded from automatic placement. The preview reports exhaustion and invalid entries rather than moving a fixed network or allocating outside the parent. IPv6 plans use prefix requirements. Standard IPv4 host-count requests use LAN rules; use explicit /31 or /32 prefixes for point-to-point or host allocations.
Document VLANs, gateways, DNS search domains and resolvers alongside allocations. Import a CSV inventory as fixed allocations, or export Summary, Allocations and Free Space sheets to Excel. Use JSON backups to transfer editable plans between devices.
For provider-specific capacity, start with the cloud subnet calculator. For exact address-list coverage rather than LAN sizing, use Range to CIDRs.