Dev

Dummy CIDR Block Generator

The dummy CIDR block generator produces randomized CIDR notation IP ranges for network configuration testing, letting you spin up realistic address blocks in seconds without touching a subnet calculator. CIDR notation combines a base IP address with a prefix length (like 10.0.0.0/16) to define how many addresses a block contains — essential for configuring VPCs, security groups, routing tables, and access control lists across AWS, Azure, GCP, and on-prem setups. When you're writing infrastructure-as-code with Terraform, Pulumi, or CloudFormation, you often need plausible IP ranges to fill out resource definitions before real network planning is complete. Hardcoding the same 10.0.0.0/8 block everywhere pollutes test outputs and masks real configuration bugs. Random CIDR blocks keep your mock configs varied and expose edge cases in parsing, validation, and overlap-detection logic. The generator supports both IPv4 and IPv6 output, covering the two address families your stack is most likely to encounter. IPv4 blocks use private RFC 1918 ranges with subnet masks from /16 to /28. IPv6 blocks follow the ULA fc00::/7 convention with prefix lengths typical of cloud provider allocations. You can generate up to dozens of blocks at once, making it straightforward to populate seed data, unit test fixtures, or demo dashboards without repetition.

How to Use

  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 modules with varied subnet CIDR blocks
  • •Seeding unit tests for IP overlap and range validation logic
  • •Generating firewall allow/deny rules for staging environment reviews
  • •Mocking AWS security group ingress and egress rule sets
  • •Creating dummy routing table entries for network simulation tools
  • •Filling Azure NSG and GCP firewall rule fixtures in demo environments
  • •Testing CIDR parsing functions across both IPv4 and IPv6 formats
  • •Building realistic network diagrams with non-repetitive address ranges

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

What is a CIDR block?

CIDR (Classless Inter-Domain Routing) notation represents an IP address range as a base address plus a prefix length, separated by a slash — for example 192.168.1.0/24. The prefix length tells you how many bits are fixed, determining the block size. A /24 gives 256 addresses; a /16 gives 65,536. CIDR replaced older class-based addressing and is now the standard across all modern networking and cloud platforms.

Are the generated CIDR blocks routable on the internet?

No. The generator uses private RFC 1918 ranges for IPv4 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and ULA fc00::/7 space for IPv6. These ranges are non-routable on the public internet, making them safe to use in test configs, documentation, and demos without risk of accidentally targeting real infrastructure.

What prefix lengths do the generated blocks use?

IPv4 blocks are generated with prefix lengths typically between /16 and /28, covering common subnet sizes used in cloud VPCs and on-prem segmentation. IPv6 blocks use prefix lengths in the /48 to /64 range, which matches typical cloud provider allocations for subnets and VNets. Both ranges are chosen to look realistic rather than trivially large or small.

Can I use these CIDR blocks in real infrastructure?

Not directly. These blocks are randomized and not checked against your existing network allocations, so overlaps with production ranges are possible. They're designed for testing and mocking only. For production use, plan your address space deliberately with a tool like an IP address management (IPAM) system or your cloud provider's VPC wizard.

Why do my generated IPv4 CIDRs start with 10, 172, or 192?

Those prefixes correspond to the three private IPv4 address ranges defined in RFC 1918: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Using private ranges ensures the generated blocks are realistic for internal network configurations while being guaranteed non-routable on the public internet.

How many CIDR blocks can I generate at once?

You can adjust the count input to generate as many blocks as you need in a single run. The default is 6, which suits most fixture or rule-set use cases. Generating a larger batch at once is useful when you need to populate seed files or test how your application handles many distinct network ranges simultaneously.

What is the difference between IPv4 and IPv6 CIDR output?

IPv4 CIDR blocks use 32-bit dot-decimal addresses (e.g. 10.34.12.0/22), while IPv6 blocks use 128-bit colon-hex addresses (e.g. fd3a:bc12:9f00::/48). The two formats are not interchangeable in most network configs. Choose the version that matches your stack — most cloud provider resources require you to specify IPv4 and IPv6 ranges separately.

How do I check if two CIDR blocks overlap?

Overlap occurs when one block's address range intersects another's. Python's ipaddress module, the ipcalc CLI tool, and online subnet calculators all support overlap detection. If you're building a network validation function and want to test it with overlapping inputs, generate a batch of blocks and manually adjust one prefix to sit inside another's range.