An overlapping subnet is a common networking issue that can prevent systems from communicating correctly. It often appears when businesses merge, new sites are connected, or networks have grown without clear planning.

Understanding what an overlapping subnet is, why it happens, and how to resolve it can help avoid connectivity problems and make future network expansion much easier.

What is an IP address?

An IP (Internet Protocol) address is a unique numerical identifier assigned to every device on a network, acting much like a physical postal address to ensure data reaches the correct destination. Operating at Layer 3 (the Network Layer), IP addresses are split into two standards: older 32-bit IPv4 addresses (e.g., 192.168.1.10) and newer 128-bit IPv6 addresses, which offer a vastly larger address space.

What is a subnet?

A subnet (short for subnetwork) is a group of IP addresses within a network. Rather than managing every device individually, organisations divide networks into subnets to improve performance, security and administration.

A small business may only need a single subnet, while larger organisations often use multiple subnets to separate departments, offices, servers or guest networks.

What is an overlapping subnet?

An overlapping subnet occurs when two different networks use the same IP address range.

This is unlikely with public IP addresses because they are allocated globally to ensure every address is unique. However, overlapping subnets are common in private networks where organisations manage their own address allocation.

For example, two separate offices may both use the private subnet: 192.168.1.0/24

If those networks are later connected together, the router cannot determine which network contains the destination address. As a result, traffic may be sent to the wrong location or fail altogether.

Why do overlapping subnets happen?

• Business mergers or acquisitions where both organisations use the same private IP ranges.
• Connecting to third-party suppliers or customers via VPNs.
• Poor network documentation.
• Networks that have expanded over time without a structured IP addressing plan.

How can you resolve an overlapping subnet?

1. Renumber one of the networks

Where possible, changing the IP address range of one subnet is the simplest and most effective long-term solution. Although renumbering requires planning and may involve updating device configurations, it removes the conflict permanently.

2. Use Virtual Routing and Forwarding (VRF)

If renumbering is not practical, Virtual Routing and Forwarding (VRF) can allow overlapping subnets to coexist.

Routers maintain routing tables that determine where traffic should be forwarded. VRF creates multiple independent routing tables on the same physical router, allowing each network to be treated as though it has its own dedicated router.

For example, one site may already use 192.168.1.0/24, while a newly acquired site uses exactly the same subnet. By placing each network into a separate VRF, both address ranges can operate independently without conflicting with one another.

VRF is commonly used where changing IP addressing is difficult or would cause significant disruption.

3. Use Network Address Translation (NAT)

In some situations, Network Address Translation (NAT) can be used to translate one network’s addresses into another range before traffic passes between them.

This can provide a practical short-term or transitional solution, although it introduces additional complexity and is generally less desirable than renumbering.

Preventing overlapping subnets

Careful planning can help avoid overlapping subnet issues before they occur. Good practice includes:

• Maintaining accurate IP address documentation.
• Using a structured IP addressing scheme.
• Reviewing address allocations before connecting new networks or sites.
• Auditing existing subnets regularly.
• Planning IP allocation as part of mergers, acquisitions and office expansions.

Summary

Overlapping subnets are usually the result of two private networks using the same IP address range. They commonly arise during mergers, acquisitions or network expansion and can lead to routing problems that disrupt communication.

Where possible, renumbering one network provides the cleanest long-term solution. When that is not practical, technologies such as VRF or NAT can allow overlapping networks to operate successfully while maintaining reliable connectivity.

If your business is currently expanding or connecting multiple sites, discover how our Managed Networks can simplify your infrastructure and eliminate routing conflicts.

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To learn more about how networks are logically segmented to keep traffic secure and organized, read our guide on What is a VLAN?

To understand how firewalls use NAT and rules to protect and control traffic leaving your local network, see our article on Why do I need outbound firewall policies?