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Dummy CIDR Block Generator

Used by developers, writers, and creators worldwide.

The dummy CIDR block generator creates randomized CIDR notation IP ranges for network configuration testing, saving you from hand-crafting subnet addresses every time you need plausible test data. CIDR notation pairs a base IP with a prefix length — like 10.0.0.0/16 — to define an address block, and it's the standard format across AWS VPCs, Azure VNets, GCP firewall rules, and Kubernetes network policies. When writing Terraform or Pulumi modules before real network planning is done, you need varied IP ranges that won't collide in test output or hide parsing bugs. This generator supports IPv4 (private RFC 1918 ranges, /16 to /28) and IPv6 (ULA fc00::/7, /48 to /64), and lets you set the count to generate up to dozens of blocks in one click.

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How to use

  1. Choose your options above
  2. Click Generate
  3. Copy your result

Detailed instructions

  1. Set the 'How Many' field to the number of CIDR blocks you need for your test fixture or config file.
  2. Select IPv4 or IPv6 from the IP Version dropdown to match your target infrastructure's address family.
  3. Click Generate to produce a fresh set of randomized private-range CIDR blocks.
  4. Copy the output list and paste it directly into your Terraform variables, firewall rule definitions, or test data files.
  5. Re-click Generate whenever you need a new non-repetitive set of blocks without editing any inputs.

Use Cases

  • •Populating Terraform VPC and subnet modules with varied IPv4 CIDR blocks before real address planning begins
  • •Seeding Jest or pytest unit tests for CIDR overlap detection and range validation functions
  • •Filling AWS security group ingress and egress rule fixtures in CloudFormation or CDK stacks
  • •Generating dual-stack IPv4 and IPv6 entries for Kubernetes NetworkPolicy test manifests
  • •Creating realistic dummy routing table entries in network simulation tools like GNS3 or EVE-NG

Tips

  • →Generate IPv4 and IPv6 blocks separately and combine them when testing dual-stack VPC configurations.
  • →If your validation logic checks for overlapping ranges, generate a large batch (20+) — random blocks occasionally overlap, giving you natural negative test cases.
  • →Prefix lengths around /24 to /26 are the most common in real cloud subnets; if your parser only sees /16s it may miss edge cases at smaller sizes.
  • →Paste generated blocks into jq or Python's ipaddress module to verify your parsing code handles all outputted formats before shipping.
  • →When mocking AWS security group rules, pair each generated CIDR with a random port range from a port generator to create fully realistic ingress/egress fixtures.
  • →Avoid reusing the same generated set across multiple test runs — commit a fresh batch per PR to prevent tests from becoming coupled to specific addresses.

FAQ

are the generated CIDR blocks safe to use in test configs and docs

Yes. The generator uses private RFC 1918 ranges for IPv4 (10.x, 172.16–31.x, 192.168.x) and ULA fc00::/7 space for IPv6 — both are non-routable on the public internet. You won't accidentally target real infrastructure by pasting these into a README, a demo dashboard, or a staging Terraform plan.

what's the difference between IPv4 and IPv6 CIDR output

IPv4 blocks use 32-bit dot-decimal addresses (e.g. 10.34.12.0/22) with prefix lengths from /16 to /28. IPv6 blocks use 128-bit colon-hex addresses (e.g. fd3a:bc12:9f00::/48) with prefix lengths in the /48–/64 range, matching typical cloud provider subnet allocations. Most cloud resources require IPv4 and IPv6 CIDR ranges to be specified separately, so use the 'Both' option when you need fixtures for dual-stack configs.

how do I test if two CIDR blocks overlap in code

Python's built-in ipaddress module handles this cleanly — network1.overlaps(network2) returns a boolean in one line. For CLI work, ipcalc and the sipcalc tool both support overlap checks. Generate a batch of blocks here, then manually adjust one prefix to nest inside another's range to create controlled overlapping test cases.