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Examples: 10.24.0.0/20 or 10.24.0.0 255.255.240.0.
Values broader than the parent normalize to its prefix; each added bit doubles the child count.
/
Use 0 to inspect address math without a host-capacity recommendation.
Rows within this count are marked planned; remaining child blocks stay available.
The prefix recommendation and fit verdict use post-reserve deployable hosts.
%
The RFC 3021 exception changes only /31 endpoint capacity.
One segment per line as label, hosts. Rows stay in entered order and align to CIDR boundaries.
Examples: VLAN, Site, Segment, or Zone.
1 suggests the first usable address.
usable slot
Choose 4 through 256 rows.
Unsupported characters are replaced in downloaded filenames only.
DecisionValuePlanning noteCopy
{{ row.label }}{{ row.value }}{{ row.detail }}
AllocationCIDRFirst usableGatewayLast usableBroadcastDeployableStatusCopy
{{ row.allocation }}{{ row.cidr }}{{ row.first_usable }}{{ row.gateway }}{{ row.last_usable }}{{ row.broadcast }}{{ count(row.deployable_hosts) }}{{ row.status }}
SegmentHostsCIDRGatewayDeployableAlignment gapStatusCopy
Add demand rows to create an ordered VLSM plan.
{{ row.label }}{{ count(row.hosts_required) }}{{ row.cidr }}{{ row.gateway }}{{ count(row.deployable_hosts) }}{{ count(row.alignment_gap) }}{{ row.status }}

The chart renderer is unavailable. Exact prefix capacities remain available in the allocation brief.

IPv4 Allocation Planner Allocate IPv4 subnets in an ordered parent CIDR, with exact usable ranges and checks for VLSM fit, alignment gaps and remaining space.
Internet Protocol version 4 (IPv4) Subnet Calculator Calculate an IPv4 subnet boundary from CIDR or a dotted mask, then check usable hosts, special-purpose ranges and reserve-adjusted capacity.
Internet Protocol version 4 (IPv4) Subnet Split Calculator Split an IPv4 parent block into equal CIDR children and check usable hosts, gateway placement and capacity fit across the complete subnet plan.
Visual IPv4 Subnet Calculator / Splitter Split an IPv4 parent block into visual CIDR leaves, rejoin aligned siblings and verify each range, usable count and complete address coverage.

IPv4 subnet planning divides one parent address block into smaller blocks whose boundaries and sizes are fixed by powers of two. A prefix such as /24 states how many of the 32 address bits identify the network. Increasing the prefix by one leaves one fewer host bit, halves each child block, and doubles the number of children available inside the same parent.

Address count and usable host capacity are not identical. An ordinary LAN subnet reserves the network and broadcast addresses, while a /32 represents one address. RFC 3021 permits both addresses in a /31 on a point-to-point link, but that exception should not be applied to a normal LAN or VLAN.

Parent block
The CIDR range that owns all child allocations.
Child prefix
The equal-sized subdivision used for the ledger.
Deployable hosts
Usable addresses left after reserve headroom is removed.
VLSM demand
An ordered list whose rows may receive different prefixes under variable-length subnet masking.

A host reserve is a planning policy, not another protocol-reserved address. It keeps part of each subnet unused for growth, but rounding matters: the reserved count is rounded up to a whole address. A subnet that can technically hold the requested hosts may therefore be marked as using reserve.

A clean allocation also needs operational context. Existing routes, DHCP scopes, static assignments, virtual networks, cloud ranges, firewalls, and overlapping acquisitions can invalidate a mathematically correct plan. The current IP address management and routing records remain the authority before deployment.

How to Use This Tool:

Begin with an owned parent block, then choose equal-child planning or add ordered demand rows when segments need different sizes.

  1. Enter Parent network in CIDR form or as an IPv4 address followed by a contiguous dotted mask. If the address is inside the block rather than on its boundary, review the normalized parent shown in the result.
  2. Set Child prefix, Usable hosts per child, Child subnets needed, and Reserve headroom. A child prefix broader than the parent is normalized to the parent prefix.
  3. Choose Host address rule. Select RFC 3021 only for /31 point-to-point links where both endpoint addresses are valid.
  4. For mixed sizes, enter one Demand list row per line as a label followed by a comma and positive host count. Rows stay in entered order and align to valid CIDR boundaries.
  5. Check Allocation brief for the overall verdict, Subnet ledger for equal children, Demand plan for VLSM placement, and Prefix capacity for the host-versus-prefix comparison.

Interpreting Results:

Ready to allocate means the requested equal children fit and each planned child meets the post-reserve host target. It does not prove the parent is unused or routable. Treat Parent block is too small and Child prefix is too small as sizing failures. Host target uses reserve means raw usable capacity is sufficient but deployable capacity after reserve is not.

  • Compare the normalized parent with the intended scope before reading any child row.
  • Check Deployable, not raw address count, when reserve is part of the policy.
  • In the demand plan, review Alignment gap and any row marked Does not fit parent.
  • Verify suggested gateways against the actual gateway convention; the entered offset is bounded to the child range.

Before changing a live network, compare every proposed range with IP address management, route tables, DHCP, cloud networks, VPNs, and firewall objects.

Technical Details:

CIDR boundaries are integer multiples of block size. The parent input is converted to an unsigned 32-bit value and rounded down to the start of its prefix. Child blocks advance from that normalized base by one child block size at a time.

Formula Core:

Prefix length controls raw block size, child count, and ordinary LAN host capacity:

Achild=232−pchild Nchildren=2pchild−pparent HLAN=Achild−2 Hreserve=⌈Husable×r100⌉ Hdeployable=max(0,Husable−Hreserve)

p is prefix length and r is reserve percentage. The ordinary LAN formula applies from /0 through /30. A /31 has zero usable LAN hosts or two RFC 3021 endpoints, and a /32 has one usable address.

Rule Core:

  1. Normalize the parent address to its CIDR boundary.
  2. Use the parent prefix whenever the requested child prefix is numerically smaller.
  3. Find the longest prefix whose deployable host count meets the requested host target.
  4. For each demand row, align the current cursor to that block size and place the row only if its broadcast address stays inside the parent.
  5. Advance the cursor after a successful placement; invalid rows do not consume space.
PrefixRaw addressesLAN usableSpecial case
/24256254Ordinary network and broadcast reservation.
/3042Small ordinary subnet.
/3120Two endpoints only in RFC 3021 mode.
/3211Single-address route or host.

The demand list keeps at most 64 non-empty rows and does not reorder them by size. A different row order can therefore produce different alignment gaps or fit outcomes even when the same demands are present.

Privacy and Planning Limits:

Address calculations run in the browser and do not query live network state. The result cannot detect existing allocations, overlaps outside the entered parent, or policy conflicts.

  • Review copied and downloaded plans before sharing because internal ranges and segment labels may be sensitive.
  • RFC 3021 mode changes only /31 endpoint capacity; it does not make /31 appropriate for LAN segments.
  • The reserve percentage is a uniform policy applied to every evaluated block, not a growth forecast.
  • A mathematically fitting plan still needs owner, routing, DHCP, security, and change-control approval.

Worked Examples:

Parent input normalized before allocation

Entering 192.0.2.5/28 normalizes the parent to 192.0.2.0/28. Four /30 children exist, but each ordinary /30 has two usable addresses and a 25% reserve removes one, leaving one deployable host. A two-host target is therefore marked Host target uses reserve, and the recommended equal-child prefix is /29.

Two point-to-point links

A 192.0.2.0/30 parent divided into /31 children produces two blocks. With RFC 3021 selected and no reserve, each block has two endpoint addresses, so two requested links fit. The same /31 plan in LAN mode would have zero usable hosts.

References: