Dev
Dummy CIDR Block Generator
CIDR notation defines IP address ranges — 10.0.0.0/16 covers addresses from 10.0.0.0 to 10.0.255.255 — and it's the standard input format for AWS VPC subnets, Azure VNets, Kubernetes NetworkPolicies, and firewall rules. When writing Terraform or Pulumi before real network planning is done, you need valid, varied IP ranges that pass syntax validation without mapping to live infrastructure. This generator supports IPv4 (private RFC 1918 ranges, prefix lengths /8 to /30) and IPv6 (ULA address space, /32 to /64), with a Both option for dual-stack fixtures. Set count to generate up to 30 blocks at once. All IPv4 output draws from the 10.x, 172.16–31.x, and 192.168.x private ranges, so no generated block can accidentally match a public host. For testing CIDR overlap detection, a large batch produces natural collisions — controlled negative test cases without manual construction.
How to use
- Choose your options above
- Click Generate
- Copy your result
Detailed instructions
- Set the 'How Many' field to the number of CIDR blocks you need for your test fixture or config file.
- Select IPv4 or IPv6 from the IP Version dropdown to match your target infrastructure's address family.
- Click Generate to produce a fresh set of randomized private-range CIDR blocks.
- Copy the output list and paste it directly into your Terraform variables, firewall rule definitions, or test data files.
- 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. IPv4 output uses private RFC 1918 ranges (10.x, 172.16–31.x, 192.168.x) and IPv6 uses ULA space — 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 notation (e.g. 10.34.12.0/22) with prefix lengths from /8 to /30. IPv6 blocks use 128-bit colon-hex notation with prefix lengths in the /32–/64 range. Most cloud resources require IPv4 and IPv6 ranges specified separately, so use Both when you need dual-stack fixtures.
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 sipcalc both support overlap checks. Generate a large batch here; random blocks occasionally overlap naturally, giving you controlled test cases without engineering collisions manually.
what does the number after the slash in CIDR notation mean
It is the prefix length — how many leading bits are fixed as the network portion. /24 on an IPv4 address fixes the first 24 bits, leaving 8 for hosts (256 addresses); a smaller number like /16 means a larger block. The generator aligns each base address to its prefix, so every block is a proper network address.
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