---
sourceDocument: Australia Telecommunications Network Inventory
sourceDocumentLink: https://servicenow-prod.fluidtopics.net/r/telecom-network-inventory

 Release :

    - australia

ft:locale :

    - en-US

ft:publication_title :

    - Australia Telecommunications Network Inventory

ft:clusterId :

    - telninv

bundleId :

    - telninv

workflow :

    - Customer and Industry


---

# Hierarchy rules for IP Subnetworks

# Hierarchy rules for IP Subnetworks {#ariaid-title1}

Release version: Australia  
Updated September 10, 2026  
![](https://www.servicenow.com/docs/portal-asset/ico-clock) 1 minute to read  
These rules apply only when creating or updating an IP Subnetwork (which always has a parent --- either an IP Address Block or another IP Subnetwork).
{#hierarchy-rules-for-ip-subnetworks__hierarchy-rules__entry__4}

| Rule | Whats checked | Rejected example | Error message |
|-|-|-|-|
| Subnetwork falls within parent | The subnetwork's CIDR is a strict subset of the parent's range. | Parent 20.0.0.0/24, child 30.0.0.0/24 | "Invalid CIDR: The current CIDR ( 30.0.0.0/24 ) is not within the parent range ( 20.0.0.0/24 )" |
| Subnetwork more specific than parent | The subnetwork's prefix is longer than the parent's. The subnetwork can't be identical to the parent. | Parent 10.10.0.0/16, child 10.10.0.0/16 | "IP Subnetwork cannot be identical to the Parent ( 10.10.0.0/16 )" |
| Unique under same parent | No other subnetwork under the same parent has the same CIDR. | Sibling 10.0.0.0/26 already exists under parent 10.0.0.0/20; new subnetwork attempts 10.0.0.0/26 | "Subnet CIDR 10.0.0.0/26 already exists under the same Parent ( 10.0.0.0/20 )" |
[ ]

{#hierarchy-rules-for-ip-subnetworks__hierarchy-rules}

## Overlap advisory {#hierarchy-rules-for-ip-subnetworks__overlap-advisory}

When a subnetwork's CIDR overlaps with --- but is not identical to --- an existing sibling under the same parent, the system displays a warning but does not block the save. The user can acknowledge the warning and
continue.

Warning text:

    Subnet CIDR ( {new CIDR} ) overlaps with existing CIDR ( {existing CIDR} ) under the same Parent ( {parent CIDR} ). Do you want to continue ?

## Uniqueness at top level --- IP Address Blocks {#hierarchy-rules-for-ip-subnetworks__uniqueness-at-top-level}

The hierarchy rules above apply to IP Subnetworks. An IP Address Block is a top-level record without a parent block, and its uniqueness is scoped by Managed Network rather than by a parent.

* If the IP Address Block has a Managed Network value, the CIDR must be unique within that Managed Network. The same CIDR may exist in different Managed Networks.
* If the IP Address Block has no Managed Network, the CIDR must be unique across all IP Address Blocks that have no Managed Network.
**Related tasks**   

* [Create an IP Subnetwork record](https://servicenow-prod.fluidtopics.net/9uvIJPRcTs8lUhUZ167X8A "Create an IP Subnetwork record to subdivide an IP Address Block or to nest a subnetwork within an existing IP Subnetwork. The subnetwork's CIDR must fall within its parent's range. This task supports both IPv4 and IPv6.")  
**Related reference**   

* [CIDR validation rules for IP Address Blocks and IP Subnetworks](https://servicenow-prod.fluidtopics.net/pz42Yup53fGYi9ouI5lr_A "The CIDR value entered on an IP Address Block or IP Subnetwork record is validated when the record is saved. Invalid CIDR values are rejected with an inline error message; the record cannot be saved until the value passes all rules.")
* [IP Subnetwork form](https://servicenow-prod.fluidtopics.net/FCJ5Na8kJ40qK1t_dzCQXQ "The IP Subnetwork form defines a subdivision of an IP Address Block or a nested subdivision of another IP Subnetwork.")
* [IP Address Block form](https://servicenow-prod.fluidtopics.net/3_uKnkKuf6RtUa~Ubb6y2w "The IP Address Block form defines a top-level IP address range using CIDR notation. Use this reference to understand the available fields and their behavior.")