CIDR Range Calculator
Enter multiple CIDR blocks to calculate IP ranges, detect overlaps, and check containment relationships.
About This Tool
The CIDR Range Calculator is a free browser-based tool that computes detailed information for one or more CIDR (Classless Inter-Domain Routing) blocks and identifies overlaps between them. For each CIDR block you enter, the tool calculates the network address, broadcast address, subnet mask, wildcard mask, first and last usable IP addresses, and total host count.
When you enter multiple CIDR blocks, the tool automatically checks for overlaps and containment relationships. It highlights which ranges conflict and explains whether one range fully contains another or if they partially overlap. You can also merge overlapping CIDRs into the smallest set of non-overlapping blocks using the supernetting / aggregation feature.
A visual IP space bar shows how your CIDR blocks relate to each other spatially, making it easy to spot gaps and overlaps at a glance. This is especially useful when planning VPC subnets, firewall rules, or IP allocation schemes.
If you need to calculate a single subnet in more detail, try the Subnet Calculator. For converting between number formats used in networking, the Number Base Converter supports binary, hex, and decimal. For DNS configuration, check out the DNS Record Generator.
All calculations run entirely in your browser. No IP addresses or network information is ever sent to any server.
How to Use
- Enter a CIDR block in the first input field (e.g.,
10.0.0.0/16). - Click Add CIDR to add more blocks for comparison and overlap checking.
- Review the calculated results for each block: network address, broadcast, subnet mask, wildcard, usable IPs, and host count.
- Check the IP Space Visualization bar to see how your ranges relate visually.
- Look for Overlap Detected alerts that highlight conflicting or contained ranges.
- View the Merged / Summarized CIDRs section to see the optimal supernet aggregation.
- Click Copy All or press Ctrl+Shift+C to copy all results to your clipboard.
Popular CIDR Examples
FAQ
What is CIDR notation?
CIDR (Classless Inter-Domain Routing) notation represents an IP address range using a base IP followed by a slash and prefix length. For example, 192.168.1.0/24 means the first 24 bits are the network portion, leaving 8 bits for host addresses (256 addresses total). This replaced the older classful addressing system and allows for more flexible subnetting.
How does overlap detection work?
The tool compares every pair of CIDR blocks by converting them to integer ranges (network address to broadcast address). If range A starts before range B ends AND range B starts before range A ends, they overlap. It then further checks if one range completely contains the other, which is a special case of overlap called containment.
What is CIDR supernetting / aggregation?
Supernetting (also called CIDR aggregation or route summarization) combines multiple contiguous or overlapping CIDR blocks into the fewest possible non-overlapping blocks that cover the same address space. This is useful for simplifying routing tables, firewall rules, and ACLs. The merged CIDRs section shows the result of this aggregation.
What is the difference between total hosts and usable hosts?
Total hosts is the mathematical count of all IP addresses in the range (2^(32-prefix)). Usable hosts subtracts 2 from this count because the first address (network address) and last address (broadcast address) are reserved and cannot be assigned to hosts. For /31 and /32 subnets, all addresses are considered usable per RFC 3021.
Can I check AWS VPC and subnet CIDRs?
Yes. Enter your VPC CIDR (e.g., 10.0.0.0/16) and your subnet CIDRs (e.g., 10.0.1.0/24, 10.0.2.0/24) to verify that subnets are properly contained within the VPC range and do not overlap with each other. The containment check will confirm the hierarchy and the overlap detection will flag any conflicts.
What is a wildcard mask?
A wildcard mask is the bitwise inverse of the subnet mask. Where the subnet mask has 1-bits (network portion), the wildcard mask has 0-bits, and vice versa. Wildcard masks are used in Cisco ACLs and OSPF area configurations. For example, a /24 subnet mask of 255.255.255.0 has a wildcard mask of 0.0.0.255.
Is my data safe?
Yes. All CIDR calculations run entirely in your browser using JavaScript. No IP addresses, CIDR blocks, or network information is ever sent to any server. You can verify this by checking the Network tab in your browser's developer tools while using the tool.
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