Allocate CIDR Blocks across AZ and Tier Rows without Overlap

How to Use This Tool

Create a sequential subnet table with alignment, capacity and overflow checks. Plan named subnet CIDRs inside an IPv4 VPC, assigning rows by AZ, tier and prefix while exposing alignment gaps, unused space and overflow conflicts.

The work this page finishes

A capacity total does not tell a cloud team which aligned block belongs to each AZ and tier, or whether ordering creates gaps and pushes the final subnet outside the VPC. The row-preserving workspace makes a correction repeatable without rebuilding a prompt or transmitting the source file.

Deterministic workflow

Normalize the VPC network boundary, allocate requested prefix blocks sequentially on their own binary boundaries, record alignment gaps and stop each row that would exceed the base CIDR. This page accepts at most 5,000 records and 2 MB of text; rejected items retain their row references while valid items remain available.

Why a dedicated interface helps

Use the table as a review artifact across AZ and tier owners; reserve future ranges deliberately and compare it with deployed infrastructure before provisioning. Immediate recalculation and a stable export are useful when the same rule must be applied consistently across a list instead of explained one item at a time.

Assumptions

• Rows are intentionally ordered by allocation priority.
• Every requested prefix is equal to or longer than the VPC prefix.
• No external network overlap is inferred.

Retain the original source until the destination accepts the result; processing stays in this tab, although the device and installed extensions remain part of the user's security boundary.

Limitations and review boundary

It models IPv4 only, does not apply AWS reserved-address usability, query an account, create route tables or guarantee that a proposed range is absent from peered networks. No supplied URL is requested and no pasted markup or code is executed; output is inserted through text-only DOM operations.

Verification and provenance

The implementation was checked against RFC 4632 — Classless Inter-domain Routing on 2026-08-26, with fixtures for a known answer, an invalid input and a batch-specific edge condition. The /24 fixture allocates four named /27 and /26 rows without overlap and exposes the addresses left after alignment.