Network · Address planning

CIDR overlap checker.

Compare two IPv4 or IPv6 CIDR blocks. See whether they collide, which contains the other, and the exact range they share.

Local-only: these may be private network ranges. Inputs are calculated in your browser and never sent to a server.

Compare CIDR blocks

Use two IPv4 blocks or two IPv6 blocks. Host bits are normalized automatically.

Result

Enter two CIDR blocks and press Check overlap.

How CIDR overlap works

A CIDR prefix fixes the most significant bits of an address. RFC 4632 defines IPv4 prefix lengths from /0 through /32; IPv6 uses the same prefix model over 128-bit addresses. This checker zeroes host bits to find each network's first address, sets them to find its last address, then compares the two closed ranges.

Two properly aligned CIDR blocks in the same address family are either disjoint, identical, or one contains the other. They cannot overlap only at one edge. IPv4 and IPv6 do not overlap because they are separate address spaces.

Specifications: RFC 4632 section 3.1 for IPv4 CIDR and RFC 4291 sections 2 and 2.3 for IPv6 addresses and prefixes.

Planning check only

This compares mathematical address ranges. It does not inspect routes, firewall rules, VLANs, VPN configuration, cloud accounts, or live reachability.