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🌐CIDR / Subnet Overlap Analyzer

Compare two lists of IPv4/IPv6 subnets to detect routing collisions, container overlaps, and duplicate CIDR blocks before connecting networks.

Presets / Examples:
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Understanding Subnet Overlaps & Routing Collisions

In IP networking, routers forward packets to their destination based on destination IP addresses and routing tables. If two networks share identical or overlapping IP address spaces (CIDR blocks), routers cannot determine where to send the traffic. This creates a routing collision (often resulting in packets being dropped or routed to the wrong destination).

This overlap analyzer compares local subnets (e.g., your on-premise data center or local VPC) against remote peer subnets (e.g., client networks, partner VPCs, or third-party integrations via VPN or VPC Peering) to detect structural conflicts.

Exact Match

Both lists contain the exact same CIDR block (e.g., 192.168.1.0/24 on both sides).

Impact: Traffic cannot be routed between these segments at all without 1:1 NAT (Network Address Translation).

A Encompasses B

Subnet A is a larger supernet containing the smaller subnet B (e.g., A is 10.0.0.0/16 and B is 10.0.1.0/24).

Impact: Subnet B hosts will not be able to reach other portions of Subnet A, or route tables will conflict due to longest-prefix-match rules.

B Encompasses A

Subnet B is a larger supernet containing the smaller subnet A (e.g., A is 172.16.1.0/24 and B is 172.16.0.0/12).

Impact: Subnet A hosts will face route blackholing or asymmetric routing when attempting to connect to the peer's networks.

Best Practices to Resolve Overlaps:

  • Longest Prefix Match Rule: Routers always prefer the most specific route (the one with the largest subnet mask/prefix length). If you route overlapping ranges, traffic to the specific subnet will override the larger route.
  • Double NAT (Network Address Translation): If re-addressing is not an option, configure Source NAT (SNAT) and Destination NAT (DNAT) on the VPN gateway to map conflicting IP ranges to unused transit subnets.
  • Plan RFC 1918 Allocation: When designing cloud VPCs (in AWS, Azure, or GCP) or office networks, pre-allocate CIDR blocks from different sub-ranges of the RFC 1918 space (e.g., assigning 10.100.0.0/16 to office A, 10.101.0.0/16 to office B) to guarantee clean, non-overlapping routing.