CIDR Calculator (IPv4 Range to CIDR Blocks)
Enter a start IP and end IP to find:
- total addresses in the range,
- the smallest single supernet CIDR that contains both endpoints,
- and the exact minimal CIDR list that covers the range precisely.
Tip: Press Enter in either field to calculate. IPv4 format only.
What this CIDR calculator does
A CIDR calculator for IP range helps you convert a plain IP interval (for example, from 10.0.0.5 to 10.0.1.250) into network blocks that routers and firewalls understand. Instead of manually guessing subnet masks, this tool computes the correct result in seconds.
In practical network engineering, this is useful for ACL creation, route summarization, cloud security group rules, and IP planning. The calculator above is designed to be operationally useful, not just educational: it returns both a broad summary CIDR and an exact block-by-block decomposition.
How to use the calculator
- Enter the first IP in your range in Start IP Address.
- Enter the last IP in your range in End IP Address.
- Click Calculate CIDR.
- Review the output table for exact CIDR blocks and coverage details.
If your range is not naturally aligned on subnet boundaries, you will usually get multiple CIDR blocks. That is expected and mathematically correct.
Key CIDR concepts for IP ranges
1) Prefix length
CIDR notation looks like 192.168.1.0/24. The /24 is the prefix length, which means 24 bits are fixed for the network and 8 bits are available for host addresses.
2) Network and broadcast
For each CIDR block, the first address is the network address and the last address is the broadcast address (in traditional IPv4 semantics). Knowing these boundaries prevents accidental overlap and rule errors.
3) Supernet versus exact cover
A single “smallest containing CIDR” (supernet) is useful for rough summarization, but it often includes extra addresses outside your exact range. For precise filtering, always use the exact CIDR list.
Why one IP range may become many CIDR blocks
CIDR blocks are powers of two and must start on alignment boundaries. Arbitrary ranges usually do not line up perfectly with those boundaries. That is why a range like 192.168.1.10 - 192.168.1.200 breaks into several blocks.
- Aligned range: fewer blocks, sometimes just one.
- Unaligned range: multiple blocks, still minimal and exact.
- Very large range: often starts and ends with smaller edge blocks and larger blocks in the middle.
Practical use cases
Firewall rules and access control lists
Security policies are cleaner and faster to audit when ranges are represented as minimal CIDR sets. This reduces duplicate or overlapping rules.
Cloud networking
In AWS, Azure, and GCP, many controls accept CIDR directly. Converting business-defined IP intervals to CIDR avoids manual mistakes and improves repeatability in infrastructure-as-code workflows.
Routing and summarization
Network teams use supernetting to reduce route table size. This calculator gives you a quick containment CIDR while still showing exact blocks when strict precision is required.
Common mistakes to avoid
- Assuming every range maps to exactly one CIDR block.
- Forgetting that a supernet may include addresses outside the requested range.
- Mixing up start/end order (start must be less than or equal to end).
- Entering invalid IPv4 octets (anything below 0 or above 255).
FAQ
Does this calculator support IPv6?
No. This page currently supports IPv4 ranges only.
Is the CIDR list minimal?
Yes. The tool uses a standard greedy method to produce the smallest number of CIDR blocks that exactly cover the selected IP range.
Can I use the output directly in ACLs?
In most systems, yes. Always verify vendor-specific syntax and whether broadcast/network addresses have special handling.
Final takeaway
A reliable CIDR calculator for IP range conversion saves time and prevents subtle networking errors. Use the exact CIDR list for precision, and use the single containing supernet when you need simplified summarization.