IPv4 Subnet Allocation Planner

Allocated Gap Free

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Parent CIDR:

Use CIDR notation such as 10.24.0.0/20; host bits are normalized to the network boundary.

Demand segments:

One line per subnet need, for example: Core services, 120, data center.

Allocation strategy:

Choose how the demand list is ordered before the planner assigns CIDR blocks.

Reservation model:

Standard reserves network and broadcast; cloud VPC reserves five addresses; point-to-point allows /31 links.

Reserve headroom: {{ reservePercentValue }}%

Use 0-90%; 15-30% is common for VLANs that are expected to grow.

0% {{ reservePercentValue }}% selected 90%

Gateway policy:

Use first usable for most VLANs, last usable for legacy networks, or none for routed-only notes.

Calculations run locally from the current fields and remain disabled until a separate enablement pass.

Smallest child subnet:

Use /30 for normal gateway-capable IPv4 LANs; /31 is only suitable for point-to-point links.

Note column:

Keep notes for handoff exports, or compact them when the ledger must stay address-only.

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IPv4 subnet allocation is the address-planning step between a parent CIDR block and real network segments. A parent range might belong to a branch office, lab, cloud VPC, campus floor, or routing domain. The allocation job is to carve that range into child subnets large enough for each segment's hosts, growth headroom, and reservation model without overlapping or wasting more space than necessary.

Classless Inter-Domain Routing (CIDR) represents each IPv4 block with a network address and prefix length. A shorter prefix such as /20 covers more addresses than a longer prefix such as /27. Variable-length subnet planning uses that fact to give large segments larger child subnets and small segments smaller child subnets, instead of forcing every segment into the same size.

Subnet size is not the same as usable host count. Under the ordinary IPv4 LAN model, the first address names the network and the last address is the broadcast address, so a /30 has four total addresses but two ordinary usable host addresses. Point-to-point designs may use /31 differently, while cloud platforms often reserve more addresses inside each subnet. Those reservation rules should be chosen before the plan is trusted.

Ordering affects waste. Allocating larger requests first usually reduces fragmentation because big blocks need stricter alignment. Preserving pasted order can still be the right operational choice when addresses must follow a building, rack, VLAN, or migration sequence, but the tradeoff should be visible. Alignment gaps are not random errors; they are the cost of keeping every child subnet on a valid CIDR boundary.

A subnet allocation plan should be reviewed before provisioning. Routing policy, existing address-management records, DHCP scopes, cloud-provider constraints, gateway conventions, security zones, and future growth can all override a mathematically valid plan. The calculation gives a structured ledger for review, not permission to deploy addresses without network-owner approval.

How to Use This Tool:

Enter the parent range and demand list first, then tune the allocation policy only when the first pass needs to match a specific network convention.

Enter the Parent CIDR, such as 10.24.0.0/20. The address is normalized to the parent network boundary before allocation.
Paste one demand segment per line as name, host count, optional note. Blank lines and comments can be left out, and the first 80 valid demand rows are planned.
Choose the allocation strategy. Largest host demand first usually reduces fragmentation; Preserve pasted order keeps operational sequence; Smallest host demand first is useful for comparison but can fragment the parent earlier.
Choose the reservation profile. Standard IPv4 LAN subtracts network and broadcast addresses through /30. Cloud VPC, five reserved subtracts five addresses per child subnet. Point-to-point friendly allows usable counts for /31 and /32 style planning.
Set reserve headroom as a percentage of requested hosts. The planner sizes each child subnet for requested hosts plus that reserve.
Choose a gateway policy: first usable address, last usable address, or no gateway suggestion. Cloud VPC mode marks the suggested first gateway address as provider-reserved.
Review Capacity Brief, Allocation Ledger, Fit Review, Address Space Map, Demand Pressure Chart, and JSON before exporting rows for network-owner review.

Interpreting Results:

Start with the fit ratio. If it reads 5/5 fit, every parsed demand row received a child subnet inside the parent. If any row is marked overflow, the current parent block, reserve percentage, max child prefix, or reservation model cannot satisfy all requested segments.

IPv4 subnet allocation result interpretation guide
Output	What it means	What to check
Capacity Brief	Parent block, usable host model, demand count, requested hosts, reserve, utilization, gaps, free tail, and gateway policy.	Confirm the parent is the intended normalized CIDR and the reservation model matches the environment.
Allocation Ledger	One row per segment with CIDR, network, first usable, gateway, last usable, broadcast, usable count, spare count, gap, note, and status.	Use the row status and spare column before copying addresses to IPAM.
Fit Review	Parser notes, fit result, ordering, headroom policy, and reservation-model cautions.	Resolve skipped demand rows or overflow rows before treating the ledger as complete.
Address Space Map	Allocated blocks, alignment gaps, and remaining free tail as parts of the parent range.	Large gaps may indicate an ordering choice or oversized child subnets.
Demand Pressure Chart	Requested hosts, reserve hosts, spare hosts, and overflow status by demand segment.	Find rows where reserve consumes most of the chosen subnet or spare capacity is tight.

A fit result is not an overlap check against your live network. It only checks the pasted demand list inside the entered parent block. Compare the ledger with IPAM, routing tables, DHCP scopes, cloud subnets, firewall zones, and any reserved ranges before using the rows.

Technical Details:

An IPv4 address is a 32-bit number. CIDR notation keeps the first prefix bits fixed and leaves the remaining bits for addresses inside the block. The planner converts dotted IPv4 addresses to unsigned integers, masks the parent down to its network boundary, calculates the parent broadcast address, and walks forward through child allocations.

Each demand row is converted to required usable hosts by adding reserve headroom. The smallest child prefix that can fit that required usable count is chosen within the selected maximum child prefix. The allocation cursor then moves to the next valid boundary for that child block. If boundary alignment skips addresses, the skipped addresses are counted as a gap. If the child block would extend beyond the parent, the row becomes overflow.

Formula Core:

\begin{array}{lcl} Total (p) & = & 2^{32 - p} \\ ReserveHosts & = & ⌈ RequestedHosts \times ReservePercent / 100 ⌉ \\ RequiredUsable & = & RequestedHosts + ReserveHosts \\ UsableLAN (p) & = & 2^{32 - p} - 2 \\ Utilization & = & AllocatedAddresses / ParentTotal \end{array}

For a segment needing 90 hosts with 20% reserve, the reserve is ceil(90 x 0.20) = 18, so the child subnet must provide at least 108 usable host slots. Under the standard LAN rule, a /25 provides 126 usable addresses and a /26 provides 62, so /25 is the smallest fitting child prefix.

Reservation Profiles:

IPv4 reservation profile rules
Profile	Usable-count rule	Planning caution
Standard IPv4 LAN	Subtracts two addresses for prefixes through `/30`; `/31` and `/32` use total count when allowed.	Use `/30` as the minimum child prefix for ordinary gateway-capable LANs.
Cloud VPC, five reserved	Subtracts five addresses from each child subnet when enough addresses exist.	Cloud providers can reserve gateway, DNS, and platform addresses that are not assignable to workloads.
Point-to-point friendly	Allows longer child prefixes for link-style planning.	Use only where device support and routing design allow `/31` or host-route style assignments.

Allocation Rules:

IPv4 subnet allocation rules and limits
Rule	Behavior	Why it matters
Parent normalization	The typed address is masked to the parent network boundary.	`10.24.1.5/20` and `10.24.0.0/20` describe the same parent block.
Boundary alignment	Each child starts at the next address divisible by its block size.	Valid CIDR boundaries can create alignment gaps between rows.
Overflow	A row that cannot fit inside the parent receives No fit values.	The demand, reserve, or parent block must change before deployment.
Demand row cap	Only the first 80 valid demand rows are planned.	Very large inventories should be split or reviewed in IPAM.

Limitations and Accuracy Notes:

The planner calculates IPv4 address ranges from the current inputs. It does not query DNS, DHCP, routers, firewalls, cloud APIs, or an IPAM database, and it does not prove that a suggested gateway is already configured or free. Use the output as a planning ledger, then validate it against authoritative network records and provider rules.

Worked Examples:

Branch office plan: A 10.24.0.0/20 parent contains core services, office users, guest Wi-Fi, printers, and network gear. Largest-first allocation places the biggest segments early, includes 20% reserve, and leaves a visible free tail for future VLANs.

Cloud VPC review: Switching to the cloud VPC profile reduces usable capacity by five addresses per child. A row that fit under the LAN model may need a larger child subnet when provider reservations are included.

Overflow repair: A demand row with 400 hosts plus 30% reserve does not fit in the remaining parent space. Increase the parent block, lower reserve, split the demand, or compare strategy modes before accepting the plan.

FAQ:

Does this allocate equal-size subnets?

No. It uses demand-driven child sizes. Each row can receive a different prefix when host demand and reserve require it.

Why are there alignment gaps?

A child subnet must start on a valid CIDR boundary for its block size. If the next free address is not aligned, addresses are skipped and counted as a gap.

Can I use public IPv4 ranges?

The arithmetic works for any valid IPv4 CIDR, and the scope label may show public or mixed scope. Routing authority and address ownership still need separate review.

Is the gateway suggestion authoritative?

No. It is a planning convention based on the selected policy. Actual gateway addresses depend on routers, cloud platform behavior, and network standards.

Glossary:

Parent CIDR: The larger IPv4 block from which child subnets are allocated.
Child subnet: A smaller CIDR block assigned to one demand segment.
VLSM: Variable-length subnet masking, where child subnets can have different prefix lengths.
Reserve headroom: Extra usable host capacity added to a demand row for growth or safety margin.
Alignment gap: Unassigned addresses skipped so the next child subnet starts on a valid CIDR boundary.

References:

RFC 4632: Classless Inter-domain Routing, RFC Editor.
RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links, RFC Editor.
Subnet CIDR blocks, Amazon Virtual Private Cloud documentation.
Azure Virtual Network FAQ, Microsoft Learn.