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Capacity chart unavailable. The same comparison data remains available as CSV.

#Child CIDRUsable rangeGatewayBroadcastUsableCopy
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This split exceeds 4,096 children, so the detailed ledger is withheld. Use the split plan and capacity curve, or choose a shallower child prefix.

Showing {{ count(ledgerRows.length) }} of {{ count(analysis.ledger_rows.length) }} child rows. Exports use the complete ledger.

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.
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 Allocation Planner Allocate IPv4 subnets in an ordered parent CIDR, with exact usable ranges and checks for VLSM fit, alignment gaps and remaining space.
IPv6 Address and Subnet Calculator Calculate an IPv6 network from an address and prefix, then review canonical boundaries, child-prefix capacity and reverse DNS or EUI-64 guidance.

Dividing an IPv4 allocation into smaller networks creates separate broadcast domains, routing boundaries, security zones, or address pools. Every extra network bit doubles the number of equal children and halves the addresses available to each child.

Classless Inter-Domain Routing (CIDR) writes the network size as a prefix such as /24. The prefix counts fixed network bits out of 32 total IPv4 bits. A longer prefix leaves fewer host bits, so a /26 is smaller than a /24 even though 26 is the larger number.

Child prefixEqual children from /24Addresses per childConventional usable hosts
/252128126
/2646462
/2783230
/306442

An entered host address first resolves to the boundary of its parent block. For example, 192.168.20.41 with /24 belongs to 192.168.20.0/24. The split begins at that network address and covers the complete parent without gaps or overlaps.

Conventional subnet planning reserves the first address as the network address and the last as the broadcast address. That leaves two fewer usable host addresses per child. Point-to-point /31 links are a separate operational convention and are outside this equal-split model, whose deepest child prefix is /30.

CIDR arithmetic does not establish that an address block is available to use. Private, shared, loopback, link-local, documentation, multicast, benchmark, and reserved ranges have different purposes. Public-looking space also requires ownership and routing approval before deployment.

How to Use This Tool:

Define the parent block first, then choose either the child prefix or the power-of-two child count that the design requires.

  1. Enter an IPv4 address and choose its Parent prefix. A host address is accepted and masked to the parent network boundary.
  2. Choose Child prefix when the subnet size is known, or Child subnet count when the number of equal zones is known. The count must be a power of two.
  3. Add Needed hosts per child when capacity must be tested. Leave it at zero for a raw address plan, and set the gateway offset to the desired usable-host position.
  4. Review the Split plan for the normalized parent, child count, capacity, gateway convention, and address scope. A negative host-fit gap means every child is short by that many usable addresses.
  5. Check the first and last rows in the Subnet ledger. If the split exceeds 4,096 children, use the plan and capacity curve or choose a shallower child prefix because the full ledger is deliberately withheld.

Interpreting Results:

Selected equal split names the normalized parent, number of children, and child prefix. A host target fits when usable hosts per child are greater than or equal to the requested count; equality is a fit with no spare usable slots.

  • Usable per child follows the conventional network-and-broadcast reservation rule. It does not subtract a gateway, DNS server, high-availability address, or other infrastructure reservation.
  • Reserved total is two addresses for every child. Finer splits increase this total even though the parent address count stays unchanged.
  • If the gateway offset is larger than the child's usable range, it is reduced to the last usable slot. Treat that as a correction to review, not confirmation that the original convention fits.
  • Address scope is a warning about the complete parent range. It does not test registry ownership, route advertisements, existing allocations, or overlap elsewhere in the network.

Technical Details:

Equal splitting borrows host bits from the parent prefix. The resulting child networks are aligned multiples of the child block size, beginning at the normalized parent network and ending at the parent broadcast address.

Formula Core

The prefix difference determines child count, while the child prefix determines addresses and conventional host capacity.

s=c-p N=2s A=232-c H=A-2

p is the parent prefix, c is the child prefix, s is the number of borrowed bits, N is child count, A is addresses per child, and H is usable hosts per child. Total usable hosts equal N times H; total reserved addresses equal 2 times N.

Splitting 192.168.20.0/24 into /26 children borrows two bits. That creates 22 = 4 children, each with 26 = 64 addresses and 62 conventional usable hosts. The first child is 192.168.20.0/26 and the last is 192.168.20.192/26.

Rule Core

RuleExact behavior
Parent rangeParent prefixes run from /1 through /29.
Child rangeThe child must be deeper than the parent and no deeper than /30.
Count planningThe requested count must be a power of two from 2 through the number of /30 children available in the parent.
Host fitUsable hosts minus needed hosts; zero is an exact fit and a negative result is a shortage.
Gateway offsetA positive requested usable slot is capped at the last usable host in each child.
Ledger limitSplits of 4,096 children or fewer receive a complete ledger; larger splits return no ledger rows.

The subnet mask keeps the first c bits set. The wildcard mask inverts those 32 bits. Each ledger row advances the network address by A, assigns the first usable address at network plus one, and assigns the broadcast address at network plus A minus one.

Limitations:

The plan creates equal, conventional IPv4 subnets only. It does not produce variable-length subnet masks, reserve infrastructure addresses beyond network and broadcast, or search existing routing and IP address management records.

  • /31 point-to-point links and /32 host routes are deliberately excluded.
  • Scope classification applies only when the entire parent falls inside a recognized special-purpose range; mixed parents are labeled public or mixed.
  • A visible ledger limit changes presentation, not the calculated child count or capacity.
  • Confirm overlaps, route policy, DHCP exclusions, high-availability addresses, and organizational reservations before deployment.

References: