# Distinct first two octets (/16 networks) count should be retrievable

Distinct first two octets (/16 networks) count should be retrievable. A Maester check for Active Directory, Sites and subnets.

Source: https://maester.dev/docs/tests/AD-SUB-09

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<div class="test-byline"><div class="test-byline-avatars"><a class="test-byline-avatar test-byline-avatar--author" href="/contributors/soulemike" title="Michael Soule · Original author"><img src="https://github.com/soulemike.png" alt="Michael Soule" width="28" height="28" loading="lazy" decoding="async" /></a></div><div class="test-byline-meta"><span class="test-byline-text">Contributed by <a href="/contributors/soulemike">Michael Soule</a></span><a class="test-byline-link" href="/contributors">All contributors →</a></div></div>

<p class="test-part">Part of <a href="/docs/tests/ad">Active Directory</a> · Active Directory · Sites and subnets · <a href="/docs/tests">All checks</a></p>

```powershell title="Run this check"
Invoke-Maester -TestId AD-SUB-09
```

## Overview

Understanding /16 network distribution helps:

- **Network planning**: Identify how many major network blocks are in use
- **Segmentation analysis**: Understand logical network boundaries
- **Capacity assessment**: Determine available /16 networks
- **Documentation**: Support network architecture documentation

Each /16 represents a major network segment (65,534 hosts).

#### Control Type

**Operational**

#### Security Recommendation

- Document the purpose of each /16 network block
- Plan /16 allocation to support organizational structure
- Reserve /16 blocks for future expansion
- Review /16 utilization during network planning

#### How the Test Works

This test extracts the first two octets from all IPv4 subnets and counts the distinct /16 networks.

#### Related Tests

- `Test-MtAdSubnetFirstOctetCount` - Analyzes first octets
- `Test-MtAdSubnetFirstThreeOctetsCount` - Analyzes /24 networks
- `Test-MtAdSubnetTotalCount` - Counts total subnets

## Details

<details>
<summary>Every attribute of this check</summary>

| Field | Value |
| --- | --- |
| Test ID | AD-SUB-09 |
| Severity | Info |
| Suite | Active Directory |
| Product | Active Directory |
| Feature | Sites and subnets |
| Area | Infrastructure hardening |
| Resource type | AdSite |
| MITRE ATT&CK | None |
| PowerShell test | [Test-MtAdSubnetFirstTwoOctetsCount](https://github.com/maester365/maester/blob/main/tests/ad/site/Test.AD-SUB-09.ps1) |
| Services | ActiveDirectory |
| Permissions | ActiveDirectory:Ldap:subnet:ReadProperty |

</details>

## Source

- Test: [`tests/ad/site/Test.AD-SUB-09.ps1`](https://github.com/maester365/maester/blob/main/tests/ad/site/Test.AD-SUB-09.ps1)
- Documentation: [`tests/ad/site/Test.AD-SUB-09.md`](https://github.com/maester365/maester/blob/main/tests/ad/site/Test.AD-SUB-09.md)