Plan with VLSM. Merge without adding addresses.
Allocate different subnet sizes, recognize overlap, and understand exact CIDR aggregation.
Start with requirements, not a fixed mask
Suppose a /24 parent must serve an office with 60 hosts, a lab with 28 hosts, and a point-to-point link with two endpoints. A /26 gives the office 62 conventional host slots. A /27 gives the lab 30. If both link devices support RFC 3021, a /31 holds its two endpoints.
The planner sorts the largest blocks first and aligns each block to its CIDR boundary. In 192.168.10.0/24, this produces 192.168.10.0/26, 192.168.10.64/27 and 192.168.10.96/31. The remaining addresses are shown as exact free CIDR blocks.
Distinguish a LAN from a point-to-point link
The planner uses traditional network and broadcast reservations for LAN rows, including small requests. A request for one or two LAN hosts therefore receives /30. Select the point-to-point checkbox only for a two-host link where /31 is supported.
The planner does not reserve a gateway separately, choose a DHCP range, configure VLANs or apply cloud-provider rules. Include gateways and other endpoints in your host requirements. It returns a complete plan only when all requested blocks fit.
Exact aggregation is not a covering route
192.168.10.0/25 and 192.168.10.128/25 exactly fill a /24, so they can merge. In contrast, 192.168.10.0/26 and 192.168.10.128/26 have a gap. A covering /24 would add addresses, so the exact merger keeps those ranges separate.
Contained entries and duplicates do not add unique addresses. The overlap count reports pairs from the input; it is not the number of distinct overlapping addresses. Host bits in a CIDR input are normalized to its network boundary and reported.
Review and reuse the output
Use the containment check to see whether an address or entire subnet is inside a parent. Use the range tool when you have explicit first and last addresses rather than a CIDR. Both IPv4 and IPv6 work, but a single operation must use one address family.
The splitter calculates the exact number of equal-sized child subnets. Large results are paginated rather than fully enumerated. A CSV export contains the displayed page; VLSM exports the complete plan. A copied plan does not change your devices or routing table.
Put it into practice
RFC 3021 · IPv4 /31 · RFC 4291 · IPv6 · RFC 5952 · IPv6 text