Detecting Overlapping CIDR Ranges

Detecting and Resolving CIDR Overlaps

Overlapping CIDR ranges are one of the most common networking mistakes, causing routing ambiguity, connectivity failures, and security blind spots. Understanding how to detect and resolve them is essential for network engineers and cloud architects.

Types of CIDR Overlap

1. Full Containment

One range completely includes another:

10.0.0.0/16  contains  10.0.1.0/24

This is expected and intentional in hierarchical designs (VPC contains subnets).

2. Partial Overlap

Two ranges share some addresses but neither fully contains the other:

10.0.0.0/23  (10.0.0.0 - 10.0.1.255)
10.0.1.0/24  (10.0.1.0 - 10.0.1.255)

Wait — this is actually full containment. True partial overlap looks like:

10.0.0.128/25  (10.0.0.128 - 10.0.0.255)
10.0.0.192/26  (10.0.0.192 - 10.0.0.255)

Again containment. Pure partial overlaps are rare with valid CIDRs because CIDR blocks always start at power-of-two boundaries. In practice, most "overlaps" are containment relationships.

Where Overlaps Cause Problems

• VPC Peering: AWS, GCP, and Azure all reject peering connections between VPCs with overlapping CIDRs
• VPN Tunnels: If your corporate network (10.0.0.0/16) overlaps with the remote site (10.0.0.0/24), routing breaks
• Kubernetes: Pod CIDR and Service CIDR must not overlap with the node network
• Firewall Rules: Overlapping rules create unpredictable allow/deny behavior

How the Calculator Detects Overlaps

The algorithm converts each CIDR to a numeric range [network_address, broadcast_address] and checks whether any pair of ranges intersects:

Range A: [start_a, end_a]
Range B: [start_b, end_b]
Overlap if: start_a <= end_b AND start_b <= end_a

If overlap exists, it further checks containment:

A contains B if: start_a <= start_b AND end_a >= end_b