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Subnet planning suite

Size IPv4 child subnets, verify usable host capacity, pack a demand list, and export allocation artifacts from one parent block.

Use the parent block you plan to split, for example 10.24.0.0/20 or 10.24.0.0 255.255.240.0.
/
The calculator shows how many child blocks fit and whether each block can hold the host target with reserve.
Set this to zero when you only need prefix math and an address ledger.
The ledger lists the first child blocks in address order and flags rows beyond this count as remaining capacity.
%
Fit status uses usable hosts minus reserve, while the tables still expose the full usable host count.
Use LAN or VLAN mode for ordinary host subnets. Use point-to-point mode when planning /31 links.
Used in ledger row names such as VLAN 01 and in copied row text.
usable slot
Adds a gateway suggestion to equal subnet and demand-plan rows without changing the address math.
Optional VLSM pack list. Enter one segment per line as label, hosts; leave blank to hide demand rows.
Large parents can contain thousands of children. This keeps the page responsive while the brief still reports the full count.
CSV, DOCX, chart, and JSON downloads use this stem with an artifact suffix.
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No rows available for this artifact.
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Tags: IP Addressing, Networking, Sysadmin
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IPv4 subnet planning turns one address block into smaller ranges that can be routed, assigned, documented, and checked without overlap. The prefix length decides how many bits describe the network and how many remain for addresses inside each subnet, so one digit can change the number of available VLANs or the host capacity of every child block.

A common planning case starts with a parent range such as 10.24.0.0/20. Splitting it into /24 children creates sixteen equal ranges, but the decision is not only the count of children. Each child also needs enough usable host addresses after network, broadcast, and reserve policy are considered.

Parent IPv4 block split into equal child subnets with a prefix boundary marker.

Good subnet plans are useful because they prevent two mistakes that are easy to miss in a spreadsheet. A child subnet can be too small after usable-host rules are applied, and a parent block can be exhausted even when the first few rows look valid. Reserve headroom adds another check by separating what fits today from what still has room to grow.

The calculated ranges are a planning aid, not a live inventory of real assignments. A subnet plan still has to be compared with router configuration, cloud network objects, DHCP scopes, firewall policy, and the address-management records used by the team that owns the block.

Technical Details:

IPv4 addresses are 32-bit numbers. CIDR notation writes the address and prefix together, so 10.24.0.0/20 means the first 20 bits identify the network and the remaining 12 bits identify addresses inside that parent block. A dotted subnet mask such as 255.255.240.0 represents the same boundary when its one bits are contiguous.

A child prefix must be equal to or longer than the parent prefix. Moving from /20 to /24 adds four subnet bits, so the parent can be divided into 2^4 equal child networks. Moving in the other direction would ask for a child block broader than the parent, so the calculation clamps that request back to the parent boundary and marks the adjustment.

Usable host capacity depends on the host address rule. LAN and VLAN subnets reserve the network address and directed broadcast address except for a single-address /32. The point-to-point option follows the RFC 3021 model for /31 links, where both addresses are usable endpoints on a two-node link.

Formula Core:

The main count formulas are powers of two because each extra prefix bit doubles the number of child blocks and halves the size of each child.

child_count = 2 child_prefix - parent_prefix raw_addresses_per_child = 2 32 - child_prefix deployable_hosts = usable_hosts - ⌈ usable_hosts × reserve_percent 100 ⌉

For a /20 parent split into /24 children, 2^(24 - 20) produces 16 child blocks. Each /24 has 2^(32 - 24), or 256 raw addresses. In LAN mode that leaves 254 usable host addresses, and a 15% reserve removes ceil(254 x 0.15), or 39 hosts, from normal deployment capacity.

Rule Core:

Planning rule Applied boundary Result consequence
Parent normalization Host-style input is rounded down to the network address for the supplied prefix or mask. Normalized CIDR and Parent network show the true base block.
Child prefix floor The child prefix cannot be numerically smaller than the parent prefix. Status can become Child prefix adjusted.
LAN host count /32 has 1 usable host, /31 has 0 in LAN mode, and other prefixes subtract 2 addresses. Usable hosts, First usable, and Last usable follow ordinary subnet rules.
Point-to-point host count /31 has 2 usable endpoints when point-to-point mode is selected. Subnet Ledger can show both addresses as endpoint addresses.
Reserve headroom Reserve hosts are rounded up from usable hosts multiplied by the reserve percentage. Deployable hosts can be lower than the raw usable count.
Demand packing Demand rows are allocated in the order entered, using the smallest prefix that meets each host target after reserve. Demand Fit Plan can report Packed, Outside parent, or a host-count correction message.

Status Boundaries:

Status Condition How to read it
Ready Requested child count fits, host target is at or below deployable capacity, and no child-prefix adjustment was needed. The selected split is internally consistent.
Reserve is tight required_hosts > deployable_hosts and required_hosts <= usable_hosts. The subnet can hold the target only by spending reserve headroom.
Child prefix too small required_hosts > usable_hosts. Choose a broader child prefix, such as /24 instead of /25.
Needs broader parent child_subnets_needed > child_count. The parent block cannot supply the requested number of child ranges.
Child prefix adjusted The requested child prefix was broader than the parent prefix. The calculation uses the parent prefix as the child prefix instead.

Everyday Use & Decision Guide:

Begin with the Parent network from the route summary, site allocation, tenant range, or lab request. If the source ticket gives a host address inside the block, enter it anyway and check the normalized CIDR badge before using the ledger. A normalized value that differs from the ticket often means the ticket used shorthand or copied the wrong mask.

Use Child prefix for the range size you plan to hand out. Equal VLAN ranges are a good fit for the Subnet Ledger, while mixed segment sizes belong in the advanced Demand list. Demand entries are packed in the order entered, so put fixed or high-priority segments first when address order matters.

  • Set Usable hosts per child to zero when you only need prefix math and address ranges.
  • Set Reserve headroom to the policy margin you want to keep out of normal use, such as 15% for growth.
  • Keep LAN or VLAN subnet active for shared networks, and use Point-to-point /31 endpoints only for two-endpoint links that support that model.
  • Use Gateway host offset to suggest a first, second, or later usable address without changing the subnet math.
  • Raise Ledger rows to render when you need more handoff rows, but rely on Allocation Brief for the full child count.

A green status does not prove that the ranges are unused. It only means the requested split, host target, reserve, and count fit within the parent according to the selected rules. Check Subnet Ledger against the real IP address management system before configuring routes, DHCP scopes, cloud subnets, or firewall objects.

When the summary reports Reserve is tight, decide whether the growth policy is too conservative or the child prefix is too narrow. When it reports Child prefix too small or Needs broader parent, fix the prefix or parent block before copying allocation rows.

Step-by-Step Guide:

Use the summary first, then move into the result tab that matches the handoff you need.

  1. Enter Parent network as CIDR or as an IPv4 address plus dotted mask. If the warning area says Check parent network, fix the IPv4 octets, mask, or prefix before continuing.
  2. Set Child prefix. Watch the summary detail for the normalized parent and selected child prefix, especially if the requested prefix was adjusted to the parent boundary.
  3. Enter Usable hosts per child, Child subnets needed, and Reserve headroom. The badges and Allocation Brief will show whether the target fits, uses reserve, or needs a broader prefix.
  4. Choose Host address rule. Use LAN or VLAN mode for ordinary subnets; choose point-to-point mode only when /31 rows should expose two endpoint addresses.
  5. Open Advanced if you need an Allocation label prefix, a Gateway host offset, a Demand list, a higher rendered row count, or a specific export filename stem.
  6. Read Allocation Brief for the parent, child count, host fit, demand-pack summary, and recommendation. A Recommendation row of Use /24 or broader means the current host target needs at least that size after reserve.
  7. Use Subnet Ledger for address ranges, Demand Fit Plan for ordered mixed-size segments, Prefix Ladder for alternative prefixes, and Address Math for mask, wildcard, first-child, last-child, and bit-boundary checks.
  8. Review Prefix Capacity Curve, Subnet Fit Stack, or Demand Fit Bars when the split needs a visual review before handoff.

Interpreting Results:

Allocation Brief carries the main decision. Check Host fit, Recommendation, and Address space used before reading individual rows. A plan can have valid-looking CIDR rows while still using reserve or requesting more child blocks than the parent can provide.

Subnet Ledger is the handoff table for equal child ranges. First usable, Gateway, Last usable, and Broadcast follow the selected host rule, so compare those fields carefully when moving between LAN mode and point-to-point /31 mode.

Prefix Ladder is the fastest way to compare alternatives. Use it when the current child prefix fails the host target or spends reserve; the row where Fit for target first changes to Fits target shows the narrowest prefix that still meets the selected host target and reserve policy.

Do not treat JSON, downloaded tables, or copied rows as approval to deploy. They preserve the calculated plan, but they do not check existing assignments, route filters, DHCP conflicts, cloud quotas, or whether a device supports RFC 3021 addressing.

Worked Examples:

  1. VLAN split from a site parent. With 10.24.0.0/20, child prefix /24, 180 usable hosts, 12 child subnets, and 15% reserve, Allocation Brief reports 16 possible child networks and 12 planned. Host fit shows 254 usable hosts, 39 reserve hosts, and 215 deployable hosts per child, so Recommendation stays at Use /24 or broader for the host target and four child ranges remain available.
  2. Small child prefix for the same host target. Keep the 10.24.0.0/20 parent but set the child prefix to /25 with the same 180-host target and 15% reserve. Host fit fails because a LAN /25 has 126 usable hosts before reserve. The summary status becomes Child prefix too small, and Prefix Ladder points back to /24 as the narrowest fitting option.
  3. Ordered demand packing. Enter demand rows such as Core services, 180, Office users, 120, Guest Wi-Fi, 60, and Network gear, 30 under a 10.24.0.0/20 parent with 15% reserve. Demand Fit Plan allocates those rows in order, using /24, /24, /25, and /26 sized blocks, and Demand Fit Bars shows required, reserve, and spare capacity for each segment.
  4. Point-to-point /31 check. Use a parent such as 10.24.100.0/30, child prefix /31, one child subnet needed, zero reserve, and Point-to-point /31 endpoints. Subnet Ledger shows two usable endpoint addresses for the first child. Switch back to LAN mode and the same /31 row has 0 usable hosts, which is the stop signal for ordinary shared-subnet planning.

FAQ:

Why did my parent network change after I entered it?

The calculation uses the true network boundary for the supplied prefix or dotted mask. Entering 10.24.3.19/20 normalizes to 10.24.0.0/20, and Allocation Brief notes that the input host was normalized.

Why does a raw address count differ from usable hosts?

Raw addresses include network and broadcast values in ordinary LAN mode. A /24 has 256 raw addresses but 254 LAN-usable hosts, and Deployable hosts can be lower again after reserve headroom is removed.

What does Reserve is tight mean?

It means the host target is at or below Usable hosts but above Deployable hosts after reserve. The subnet can hold the hosts, but it spends part of the headroom you said should stay unused.

Why did a demand row fail?

A demand row can fail when the host count is missing or not positive, when no IPv4 prefix can fit the requested hosts, or when the next aligned block would exceed the parent. Check the row status in Demand Fit Plan and adjust the host count, order, parent, or reserve value.

Does the ledger know which ranges are already assigned?

No. Subnet Ledger calculates possible ranges inside the parent block. It does not query IPAM, routers, DHCP servers, cloud networks, or firewall objects, so the final range list still needs an operational conflict check.

Are my subnet entries uploaded?

The subnet calculation and table exports are produced in the browser session. Copy Link places current parameters in the URL, so treat copied links as sensitive when they describe private network plans.

Glossary:

CIDR
Address notation that pairs an IPv4 address with a prefix length, such as 10.24.0.0/20.
Parent network
The larger IPv4 block that is split into child ranges.
Child prefix
The prefix length used for each equal child subnet in the ledger.
Usable hosts
Host addresses available after applying the selected LAN or point-to-point rule.
Reserve headroom
The portion of usable hosts held back before the deployable capacity verdict is calculated.
Wildcard mask
The inverse of the subnet mask, often used in access-list and routing policy notation.
Alignment gap
Address space skipped so an ordered demand row can start on the required prefix boundary.
RFC 3021
The point-to-point IPv4 rule that allows both addresses in a /31 link to act as usable endpoints.

References: