Allocate by requirement
VLSM uses different prefix lengths within one parent network. Start with the largest requirement so that smaller allocations do not fragment the available space. Record the required hosts rather than assuming every subnet needs the same size.
Read the worked example
A /25 contains 128 addresses and traditionally allows 126 IPv4 hosts. A /26 allows 62 and a /27 allows 30. The illustrated allocations fit within 10.0.0.0/24 without overlapping, leaving the final /27 unallocated.
Keep IPv6 distinct
IPv6 planning uses prefix requirements and has no broadcast address. Do not carry IPv4 host-subtraction rules into IPv6. Choose prefixes appropriate to the intended network design; the calculator’s address count is not a statement about routability.
Example
Worked example: parent 10.0.0.0/24 100 hosts -> 10.0.0.0/25 50 hosts -> 10.0.0.128/26 20 hosts -> 10.0.0.192/27 Remaining: 10.0.0.224/27
What to check next
- Enter a private or public parent CIDR in Subnet Planner.
- Add each subnet requirement and review the largest-first allocation.
- Check ranges and free space with Subnet Calculator, then export the table for your network plan.