{
  "id": 2860551,
  "title": "Auditing an Existing Subnet Plan Before a Cloud Migration",
  "url": "https://urgent.news/2026/08/23/auditing-an-existing-subnet-plan-before-a-cloud-migration",
  "topic": "tech",
  "section": "Tech",
  "published": "2026-08-23T20:03:03.000Z",
  "source": {
    "name": "Dev.to",
    "slug": "dev-to",
    "url": "https://dev.to/lizely/auditing-an-existing-subnet-plan-before-a-cloud-migration-2f6g"
  },
  "original_language": "en",
  "account": "Existing subnet plans often start from a Confluence page, router export, and a network engineer who cannot remember all the subnets in use. The goal is to prove that every subnet still fits its purpose and does not overlap with others before migrating to the cloud. The audit process involves gathering all router, switch, and firewall interface descriptions, DHCP scopes, DNS zone files, cloud account ranges, VPN configurations, and documenting the last time each subnet was touched.\n\nFirst principles subnet math is crucial. For each row in the inventory spreadsheet, calculate the network address, broadcast address, usable host range, and prefix length. Subtract allocated subnets from the parent block and identify free CIDR ranges for migration. Perform these calculations manually for the top five entries to ensure reliability.\n\nCommon audit failure modes include overlapping allocations from acquisitions, the \"only a few hosts\" myth, documentation lag, and asymmetric VPN ranges. Address these issues by identifying parent blocks, right-sizing subnets, cross-checking against DHCP leases and firewall logs, and ensuring site allocations are distinct.\n\nDuring the audit meeting, follow a practical checklist: merge inventory sources into one sheet, verify each row's purpose and owner, re-derive CIDR math, perform overlap and containment tests, right-size prefix lengths, and confirm cloud ranges are unique. Sign off on the audit with a reviewer's name and date.\n\nFor the new cloud ranges, implement a prefix budget by reserving contiguous /16s per region or business unit. Within each /16, allocate /20s for infrastructure, shared services, and application subnets. This approach ensures clean summary routes, easier capacity planning, and straightforward audit answers. Additionally, consider IPv6 prefixes for each VPC to future-proof the architecture.",
  "summary": "Most subnet articles start from a blank piece of paper and end with a clean diagram. That is not the situation most teams are actually in. The situation is a Confluence page from 2019, a router config someone exported last quarter, and a network engineer who swears \"the 10.40.0.0/16 is in use somewhere\" but cannot remember where. The migration deadline is in three weeks. Your job is to prove, on…",
  "key_points": [
    "Audit existing subnet plan before cloud migration to ensure no overlaps.",
    "Perform subnet math manually for top five entries to verify calculations.",
    "Implement prefix budgeting with contiguous /16s per region for cloud ranges."
  ],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}