CIDR Overlap Checker

To check whether two CIDR blocks overlap, compare their first and last IP addresses. If any part of the address ranges intersects, they overlap. For example, 192.168.1.0/24 overlaps with 192.168.1.128/25 because the /25 is inside the /24. But 192.168.1.0/25 and 192.168.1.128/25 do not overlap; they are adjacent. This check is important before assigning VLAN subnets, adding firewall rules, or migrating routes. A clean overlap result prevents duplicate addressing problems that can be painful to trace later.

A CIDR block is inside another block when every address in the smaller block fits within the larger block's range. For example, 10.10.10.64/26 is inside 10.10.10.0/24. The /26 has fewer addresses and a longer prefix length, so it is more specific. Do not judge this only by the first few octets, because boundaries matter. The calculator should show containment clearly. In real work, this helps verify whether a planned subnet belongs to the approved site, VRF, cloud VPC, or summarization range.

Overlapping CIDR ranges are risky because two parts of the network may believe they own the same addresses. That can break routing, NAT, VPN tunnels, firewall rules, and troubleshooting. For example, if a branch office and a cloud VPC both use 10.0.1.0/24, a VPN route may not know where traffic should go. Overlap is also a common problem during mergers and cloud migrations. Check overlap early, before building tunnels or ACLs, because fixing the addressing plan later is usually more disruptive.

To find the shared address range between CIDRs, convert each CIDR to a start IP and end IP, then take the later start address and the earlier end address. If the later start is less than or equal to the earlier end, that interval is the overlap. For example, if one range ends at .255 and the other starts at .128, the shared part begins at .128. A calculator does this quickly, but the logic is worth learning because it explains why some ranges touch but do not actually overlap.

Overlap checking is very useful for firewall rules because broad rules can silently include addresses covered by another rule. If two CIDRs overlap, rule order and device behavior may decide which policy wins. For example, a deny rule for 10.1.0.0/16 and an allow rule for 10.1.5.0/24 need careful ordering on many platforms. Before cleanup, compare the CIDRs, identify shared ranges, and document the intended policy. This avoids the common mistake of deleting a rule that looked duplicate but had a specific security purpose.

Overlapping CIDRs can cause routing conflicts, but the exact effect depends on the routing table. Routers use longest-prefix match, so a more-specific prefix usually wins over a broader one. For example, 10.0.0.0/8 and 10.1.2.0/24 can coexist, and traffic to 10.1.2.10 follows the /24 route. The problem starts when the overlap is accidental or points to the wrong next hop. In VPNs, clouds, and multi-site networks, overlapping private ranges often create unreachable hosts or asymmetric paths.

Before assigning a new subnet, check it against the existing IP plan, routing table, DHCP scopes, VPN ranges, and cloud networks. Enter the proposed CIDR and compare it with current allocations. If the tool shows overlap or containment, stop and review before deployment. A subnet may look free in one spreadsheet but already be used in another site or tunnel. A useful practice is to record network address, mask, gateway, VLAN, location, and owner so future overlap checks are not guesswork. That small habit saves time.