Can two CIDR blocks partially overlap?

No. Because CIDR blocks are aligned to their size, two blocks are either disjoint, identical, or one fully contains the other.

Why do overlapping VPC CIDRs matter?

Routing needs each destination to be unambiguous. Cloud providers such as AWS do not allow VPC peering between VPCs with overlapping CIDR blocks, and overlaps also break site-to-site VPN routing.

What does it do with a block like 192.168.1.17/24?

Host bits are cleared and it is treated as the network 192.168.1.0/24, with a warning so you can fix the source.

CIDR Overlap Checker - IPv4 & IPv6 Ranges

Detect overlapping or contained IPv4 and IPv6 CIDR ranges, e.g. across VPCs and subnets.

About CIDR Overlap Checker

Paste a list of IPv4 and IPv6 CIDR blocks, one per line with an optional label, and find every pair that overlaps. Each conflict is described as identical or one block containing the other, with the overlapping range and its size; up to 16 blocks also get an overlap matrix. The tool also outputs the aggregated list: the minimal set of CIDRs covering the same addresses with overlaps removed and adjacent blocks merged. Lines starting with # are ignored.

Common uses

Check that VPC or VNet ranges don't overlap before setting up peering or a VPN
Audit firewall or security group rules for redundant entries
Merge an allowlist of CIDR blocks into the shortest equivalent list
Verify that a new subnet fits without colliding with existing ones

FAQ

Can two CIDR blocks partially overlap?: No. Because CIDR blocks are aligned to their size, two blocks are either disjoint, identical, or one fully contains the other.
Why do overlapping VPC CIDRs matter?: Routing needs each destination to be unambiguous. Cloud providers such as AWS do not allow VPC peering between VPCs with overlapping CIDR blocks, and overlaps also break site-to-site VPN routing.
What does it do with a block like 192.168.1.17/24?: Host bits are cleared and it is treated as the network 192.168.1.0/24, with a warning so you can fix the source.