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CIDR Overlap / Subnet Conflict Checker

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ガイド

CIDR Overlap / Subnet Conflict Checker

Paste a list of CIDR ranges and instantly see which ones overlap. The checker performs exact bitwise comparison of every pair of subnets — for both IPv4 and IPv6 — and reports the precise conflicting address range and how many addresses collide. It is built for network engineers planning VPC peering, multi-region address space, firewall rules, or VPN routes, where a single overlapping subnet can silently break routing.

使用方法

Enter one CIDR range per line, e.g. 10.0.0.0/16 または 2001:db8::/32.
Results update automatically as you type or paste.
Review the summary counts, then inspect each conflicting pair in the table below.
For every conflict you see the relationship (identical, containment, or partial overlap), the exact overlap range, and the number of overlapping addresses.

機能

IPv4およびIPv6のサポート – mixed lists are handled; only same-family ranges are compared.
Exact overlap range – shows the first and last colliding address, not just a yes/no.
Relationship detection – flags identical ranges, full containment, and partial overlaps.
Address counts – reports how many addresses overlap, with power-of-two notation for huge IPv6 blocks.
100%クライアントサイド – all math runs in your browser; no IP data is sent anywhere.

 よくある質問

What does it mean for two CIDR blocks to overlap?

Two CIDR blocks overlap when at least one IP address belongs to both ranges. Every CIDR block represents a contiguous span of addresses from a network base to a broadcast/last address. Two spans overlap whenever the start of one is less than or equal to the end of the other and vice versa. Overlap ranges from a full duplicate to one block being entirely contained inside another to a partial intersection at the edges.
Why are overlapping subnets a problem in network design?

Routers and routing tables assume address ranges are unambiguous. When two subnets overlap, a destination IP can match more than one route, so longest-prefix-match rules decide the winner and traffic may be sent to the wrong network. In cloud environments overlapping CIDRs block VPC peering and VPN tunnels outright, because the platform cannot decide which side owns a shared address. Detecting overlaps before deployment avoids silent black-holing of traffic.
How is a CIDR prefix length related to the number of addresses?

The prefix length is the count of leading bits fixed as the network portion. The remaining bits are host bits. An IPv4 address has 32 bits, so a /24 leaves 8 host bits and 256 addresses, while a /16 leaves 16 host bits and 65,536 addresses. IPv6 uses 128 bits, so a /64 still leaves 64 host bits, which is roughly 1.8 x 10^19 addresses. Each step smaller in prefix length doubles the size of the block.
What is the difference between containment and partial overlap?

Containment means one block fits entirely within another, for example 10.0.1.0/24 sits inside 10.0.0.0/16. Partial overlap means two blocks share some addresses but each also has addresses the other does not, which usually signals misaligned boundaries. Identical blocks are a special case where both ranges are exactly equal. Distinguishing these helps decide whether to renumber, resize, or simply remove a redundant entry.