Internet Protocol version 4 (IPv4) Subnet Split Calculator

IP address:

Enter any IPv4 host in the parent block, e.g. 192.168.10.45.

Current CIDR:

Select the existing parent prefix before you carve equal child subnets.

/1 broad parent /{{ cidr }} selected /29 tight parent

Planning mode:

Use child CIDR for mask-first planning; use count for powers-of-two targets.

Target CIDR:

Choose a deeper prefix through /30; each option shows the child count.

/{{ cidr + 1 }} larger children /{{ cidr_new }} selected /30 smallest children

Child subnet count:

Only powers of two that fit the selected parent block are available.

fewer children /{{ selectedTargetPrefix }} selected more children

{{ error }}

Needed hosts per child:

Whole number of usable hosts required per child subnet; use 0 for none.

hosts

Gateway host offset:

1 = first usable host, 2 = second; capped automatically for small child subnets.

usable slot

Preview rows:

Show the first 32-512 child subnets in the on-page ledger.

CSV header row:

Turn off only when appending rows into a file that already has headers.

CSV filename:

Use a short filename such as branch-a-subnet-ledger.csv.

{{ notice }}

Section	Field	Value	Detail	Copy
{{ row.section }}	{{ row.label }}	{{ row.value }}	{{ row.detail }}

#	Network	CIDR	Usable Range	Gateway	Broadcast	Usable	Copy
{{ row.index }}	{{ row.networkAddress }}	{{ row.cidr }}	{{ row.usableRange }}	{{ row.gatewayAddress }}	{{ row.broadcastAddress }}	{{ row.usableCount.toLocaleString() }}
Detailed child rows are capped Use the planner table and forecast chart, or choose a shallower split to render a ledger.

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Equal IPv4 subnet splitting is useful when one parent range has to become several same-size networks. The parent may come from an office allocation, a lab worksheet, a tenant pool, or a routed branch block, but the planning question stays concrete: how many child CIDR ranges can be carved out, and how many usable host addresses will each child keep?

An IPv4 CIDR block is written as an address plus a prefix length, such as 192.168.20.0/24. The prefix length says how many of the 32 address bits identify the network. A shorter prefix covers more addresses, while a longer prefix leaves fewer host bits and creates a smaller range. Equal splitting moves that boundary to the right by a whole number of bits, so child counts grow as powers of two.

Parent block: The original CIDR range being divided, normally chosen from an address plan, routing allocation, or lab worksheet.
Child subnet: One equal-size network created from the parent by borrowing one or more host bits.
Usable hosts: The ordinary host count after reserving the network address and broadcast address at the edges of each child subnet.
VLSM: Variable Length Subnet Mask planning, used when child networks need different sizes instead of one repeated prefix.

Equal subnet split diagram showing one IPv4 /24 parent becoming four /26 child subnets with 62 usable hosts each.

Borrowing one host bit doubles the child count and cuts each child range in half. Borrowing two host bits creates four equal children, borrowing three creates eight, and the pattern continues until the chosen child prefix is reached. The count can look attractive on paper, but the usable-host pool shrinks quickly because every child repeats its own network and broadcast addresses.

Equal splits fit repeated patterns best: four same-size office VLANs from one branch range, eight identical lab benches from one test block, or a group of tenant networks that should all receive the same prefix. They are a poor match when one segment needs far more hosts than the others. In that case, a variable-length subnet masking plan or a larger parent range prevents wasted addresses and undersized children.

Common equal subnet split choices and their tradeoffs
Planning choice	What changes	Tradeoff
Move one prefix bit deeper	The child count doubles.	Each child has half the raw address span of the parent step before it.
Choose child count first	The count maps back to the matching child prefix.	Only powers of two are valid for equal CIDR splits.
Choose child CIDR first	The child prefix fixes both count and capacity.	A deeper prefix may produce many rows while leaving too few hosts per child.
Set a gateway offset	The suggested gateway moves within each child usable range.	The offset must stay inside the child host span and may be clamped for small children.

A syntactically valid IPv4 split is still only part of an address plan. Private RFC 1918 space, carrier-grade NAT space, documentation blocks, loopback, link-local, multicast, benchmark, and reserved ranges have different routing meanings. A usable count also says nothing about whether the range is already assigned, allowed by a firewall policy, or suitable for the gateway convention used by a specific network.

How to Use This Tool:

Start by making the parent block correct. Every child range, host count, gateway suggestion, and exported row depends on that boundary.

Enter any address inside the range in IPv4 address. The Parent Block result will mask it back to the network boundary, so 192.168.20.41 with /24 becomes 192.168.20.0/24.
Set Current CIDR to the parent prefix. The available parent choices leave room for at least one deeper child prefix through /30.
Choose Planning mode. Pick Choose child CIDR when the child mask is already known, or Choose child subnet count when the requirement is a power-of-two number of equal networks.
Use Needed hosts per child when capacity matters. A value of 0 reports raw split arithmetic; any positive value adds a spare-or-short Capacity Verdict.
Set Gateway host offset only if the suggested gateway should not be the first usable address. If the offset exceeds the child usable span, the page reduces it and shows a warning.
Review Split Summary and Split Planner before using rows. Confirm the parent network, child prefix, child count, host-fit value, reserved-address count, and first and last child CIDR values.
If Subnet Ledger is available, use it for exact network, usable range, gateway, broadcast, and per-row copy actions. For very large splits, rely on the planner and Split Forecast Chart for sizing, or choose a shallower child prefix when a full row ledger is required.

Interpreting Results:

Read the normalized Parent Block first. If that value is not the block you meant to divide, the child rows will be consistently wrong even though their arithmetic is valid. Fix the address or Current CIDR before checking capacity.

The next check is whether the child layout still leaves enough usable host addresses. A split can produce the requested number of child subnets and still be too small for the endpoints, gateway, static reservations, or growth allowance that each child needs.

IPv4 subnet split result interpretation guide
Result	Check	Why it matters
Parent Block	The network boundary implied by the entered address and parent prefix.	Typing a host address is fine, but the split must start from the intended parent network.
Chosen Child Layout	The child prefix, child count, first and last child CIDR, and gateway slot.	Edge CIDRs catch off-by-one planning mistakes before rows are copied into IPAM.
Capacity Verdict	Spare or short usable hosts per child against Needed hosts per child.	A negative value means the selected child prefix is too deep for the requested host target.
Subnet Ledger	Exact network, usable range, gateway, broadcast, and usable count for each rendered child.	Rows are hidden above the large-split cap, so a full handoff may require a shallower split.
Split Forecast Chart	Usable hosts per child and child count across available deeper prefixes.	The deepest fitting prefix is useful only when its host-fit value is zero or positive.

A positive host-fit value does not prove that a range is unassigned, routable, or allowed by policy. Review warnings about special-use ranges, gateway clamping, and capped ledgers before handing off the plan. Copied links preserve the current inputs and active result tab, so treat them as address-plan data rather than public notes.

Technical Details:

Classless Inter-Domain Routing, or CIDR, represents an IPv4 range as a 32-bit address and a prefix length. The prefix bits are the fixed network portion. The remaining bits are host bits, and those host bits are the only space available for equal subnet splitting.

Equal splitting is deterministic because each additional prefix bit creates one binary branch. Moving from /24 to /25 borrows one host bit and produces two children. Moving from /24 to /26 borrows two host bits and produces four children. Child networks stay aligned because each row advances by the child block size.

This planner uses the conventional host-pool model for children, so child prefixes stop at /30. The /31 point-to-point interpretation is useful in routing design, but it would change the host-count rule and gateway assumptions for every child row.

Formula Core

The governing math is prefix arithmetic plus the conventional IPv4 network-and-broadcast reservation.

\begin{array}{lcl} N & = & IP \land M_{p} \\ B & = & P_{c} - P_{p} \\ C & = & 2^{B} \\ S & = & 2^{32 - P_{c}} \\ U & = & S - 2 \\ F & = & U - H \end{array}

IPv4 subnet split formula symbol definitions
Symbol	Meaning	Result field or input
`N`	Parent network address after host bits are cleared by the parent mask.	Parent Block
`Pp` and `Pc`	Parent prefix and child prefix.	Current CIDR and Target CIDR
`B`	Borrowed host bits.	Split bits borrowed
`C`	Number of equal child subnets.	Child subnets
`S` and `U`	Total child addresses and usable hosts per child.	Usable per Child
`H` and `F`	Requested hosts per child and spare-or-short host fit.	Needed hosts per child and Capacity Verdict

A /23 parent split into /26 children borrows three bits, so C = 2^3 = 8. Each child has 2^(32 - 26) = 64 total addresses and 62 usable hosts. With a 50-host target, the host-fit value is 62 - 50 = 12 spare usable addresses per child.

Validation and boundary rules for the IPv4 subnet split calculator
Boundary	Applied rule	Planning effect
IPv4 input	Dotted-decimal octets must be between `0` and `255`.	Invalid addresses stop the calculation before rows are shown.
Parent prefix	Current CIDR is limited to `/1` through `/29`.	Every parent option leaves at least one deeper child prefix through `/30`.
Child prefix	Children must be deeper than the parent and no deeper than `/30`.	The usable-host model stays with network and broadcast reservations.
Child count	Count mode exposes powers of two that map to valid deeper prefixes.	Non-power-of-two equal splits are not CIDR-aligned.
Gateway slot	The gateway offset is clamped to the child usable range.	A suggested gateway does not land on the broadcast address or outside the child subnet.
Detailed ledger	Detailed child rows are withheld above `4,096` children.	Large splits remain visible in summary and forecast form without rendering an oversized table.

The netmask and wildcard mask are complementary dotted-decimal views of the same prefix. A /26 child has netmask 255.255.255.192 and wildcard mask 0.0.0.63, which is useful when firewall or access-control syntax describes the variable bits instead of the fixed bits.

Prefix Comparison

For any fixed parent block, deeper child prefixes increase route count and reduce usable hosts. When a host target is present, the deepest fitting prefix is the largest child count whose usable-host count is still greater than or equal to the target.

Example equal split prefix comparison for a /24 parent
Child prefix	Child count	Total per child	Usable per child	Planning note
`/25`	2	128	126	Two larger child networks.
`/26`	4	64	62	Four medium child networks.
`/27`	8	32	30	More segmentation with smaller pools.
`/30`	64	4	2	Smallest child size in this conventional model.

Address Scope Flags

Subnet math does not decide whether an address block should be deployed. The parent network is also classified so special-use ranges are not mistaken for ordinary routable host pools.

IPv4 address scope labels used when reviewing subnet split results
Scope label	Examples	Meaning for a split plan
Private RFC 1918	`10.0.0.0/8`, `172.16.0.0/12`, `192.168.0.0/16`	Suitable for internal plans when local routing, NAT, and overlap policy allow it.
Carrier-grade NAT	`100.64.0.0/10`	Shared address space, not ordinary private LAN space.
Documentation or benchmark test	`192.0.2.0/24`, `198.51.100.0/24`, `203.0.113.0/24`, `198.18.0.0/15`	Useful for examples, lab notes, or tests, but not live host allocation.
Loopback, link-local, multicast, or reserved	`127.0.0.0/8`, `169.254.0.0/16`, `224.0.0.0/4`, `240.0.0.0/4`	Not normal routed host-address pools for equal subnet deployment.
Public or other unreserved	Ranges outside the flagged categories	May be publicly routable, but ownership and routing policy still need external verification.

Limitations and Privacy:

Equal split arithmetic proves boundaries and capacity under the selected parent and child prefixes. It does not check live network ownership, IPAM records, DHCP exclusions, routing policy, or whether a copied gateway convention matches a specific environment.

Calculations run in the browser; there is no remote lookup for entered address-plan values.
Copied scenario links can include the entered IPv4 address, prefixes, host target, gateway offset, and active tab in the page URL.
Detailed child rows are intentionally withheld above 4,096 children; summary, forecast, and JSON output remain available.
Scope labels identify built-in special-use categories and do not prove public reachability, ownership, or policy approval.

Worked Examples:

Four same-size VLAN ranges from one /24

Enter 192.168.20.41, set Current CIDR to /24, and choose Target CIDR /26. Parent Block becomes 192.168.20.0/24. Chosen Child Layout reports four child subnets with 62 usable hosts each, from 192.168.20.0/26 through 192.168.20.192/26.

Planning by child count

Enter 10.12.0.18, set Current CIDR to /23, choose Choose child subnet count, and select 8 child subnets. The matching child prefix is /26. With Needed hosts per child set to 50, Capacity Verdict shows +12, so every child has 12 spare usable host slots before local reservations beyond the standard edge addresses.

Catching a child that is too small

Use 198.51.100.20 with Current CIDR /28 and Target CIDR /30. The parent normalizes to a documentation range, and each child has only two usable hosts. If Needed hosts per child is 3, Capacity Verdict shows a one-host shortfall. Move shallower, pick a larger parent, or switch to a different planning model instead of treating the row ledger as deployable.

FAQ:

Can I enter a host address instead of the network address?

Yes. Any valid IPv4 address inside the parent block is accepted. Parent Block shows the masked network boundary that the split actually uses.

Why are child-count choices powers of two?

Equal CIDR splits borrow whole host bits. Each borrowed bit doubles the child count, so valid equal counts are powers of two relative to the parent prefix.

Why does the child prefix stop at /30?

The calculator uses the conventional IPv4 host model, where each child has a network address and a broadcast address. A /31 point-to-point interpretation is outside the shown host-count model.

Can the split make differently sized child subnets?

No. Every child row uses the same prefix and block size. Use a VLSM-oriented planner when one child needs a larger pool than another.

Why did the detailed subnet ledger disappear?

Splits above 4,096 child subnets intentionally hide the detailed ledger. Split Planner and Split Forecast Chart remain available, but a shallower split is needed when you need every child row rendered.

Do calculations leave the browser?

The split, ledger, forecast chart data, and JSON are calculated in the browser. A copied scenario link can include the entered address-plan values in the URL, so share it with the same care as the exported plan.

Glossary:

Parent block: The original IPv4 network range being divided into equal child subnets.
Child subnet: One equal-size CIDR block created by borrowing host bits from the parent.
Split bits: The difference between the child prefix and the parent prefix.
Gateway host offset: The usable-host slot used for the suggested gateway address inside each child subnet.
Host-fit value: The spare or short usable-host count per child after comparing capacity with the requested host target.
Wildcard mask: The inverse of a subnet mask, often used when route or firewall rules describe variable bits.

References:

RFC 4632: Classless Inter-domain Routing (CIDR), RFC Editor, August 2006.
RFC 950: Internet Standard Subnetting Procedure, RFC Editor, August 1985.
RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links, RFC Editor, December 2000.
RFC 1918: Address Allocation for Private Internets, RFC Editor, February 1996.
RFC 6598: IANA-Reserved IPv4 Prefix for Shared Address Space, RFC Editor, April 2012.
IPv4 Special-Purpose Address Space Registry, IANA.