Multi-cloud architectures: What is IP overlap, and how can it be addressed?

Organizations are increasingly moving to hybrid cloud or multi-cloud architectures to gain the flexibility and performance needed for digital transformation. While a hybrid cloud solves many challenges, it can also add a new issue—IP overlap, which can cause errors with sending and receiving data.

IP overlap occurs when multiple devices that connect or communicate with each other use the same IP address. A common cause of IP overlap is when companies use non-routable address space or outdated “1918” networks. When a hybrid cloud connects multiple networks, some networks may include the same IP addresses. Many networks use the same IP address because there is no standard practice for assigning IP addresses in multi-cloud architectures and multiload environments. When each cloud environment uses the same “1918” range, IP overlap occurs when combining the different environments. This is not an issue when the networks stay separate and do not communicate, but interconnectivity is becoming increasingly more common.

IP overlap, which is also called network overlap, does not typically occur on a single network that does not interact with other networks. Two networks must merge or interconnect after implementation for IP overlapping to happen. Sometimes the issue can also occur when using a public cloud—especially a large cloud service provider known as a hyperscaler. With the emergence of more ways to connect and set up networks, such as site-to-site VPNs, IP overlap has become more common.

Many organizations face this issue when merging with or acquiring another company, as both companies may use the same range of IP addresses. Particularly, if both organizations have a lot of locations or devices on their networks. IP overlapping also commonly occurs when working with multiple vendors or offering services to customers. While many organizations assume using a private IP network solves the problem, it's still possible that the network may connect to another network with the same IP address in the future.

Many organizations don't realize they have an IP overlapping issue until a problem occurs. It's also often difficult to pinpoint if an issue is related to an overlapping IP address or caused by one of the many other possible network issues, which means IT departments have to spend considerable time troubleshooting.

Two common issues with IP overlapping are:

• Devices and subnets are unable to communicate. Because the IP address is how subnets and devices communicate with other points, two subnets with the same IP address cannot communicate. This problem becomes much more complicated to track down and fix if one or both of the addresses include network services.
• Data sent to the wrong subnet. When a piece of traffic is addressed to an IP address experiencing IP overlapping, the network doesn't always send it to the correct location. Because both locations are connected to the network, it appears that the traffic has been correctly delivered. This situation can cause many issues, ranging from security to operations interruption.

Organizations typically take a reactive approach to the overlap issue by trying to fix the problem once it occurs. At this point, however, the overlap is already creating issues and many times the issue could be with more than one IP address. Taking a proactive approach is more likely to prevent the problem from ever happening. For example, look for IP overlapping issues when facing a situation where two networks are being connected, such as a merger or new vendor relationship.

Here are three ways to solve and prevent IP overlapping:

1. Renumber your network. One way to solve IP overlapping is to renumber all the IP addresses in your network after a merger. Because it is a permanent solution and can be managed in-house, this approach also helps save money and makes troubleshooting easier in the future by eliminating IP overlap as an issue. However, in many scenarios where IP overlapping happens, such as working with customers and vendors, this solution is not feasible because you do not control those networks and cannot renumber them.
2. Implement network address translation (NAT). Once IP overlapping is discovered, you can use NAT techniques to connect specific IP addresses to the internet. This is a good short-term fix. However, because it is a workaround approach, the problem often reappears. NAT can also quickly increase operational costs for both troubleshooting and network management because the issues may begin again. Additionally, NAT requires additional administrative and maintenance time.
3. Use a common Dynamic Host Configuration (DCHP) server. When you use a DHCP server, the protocol dynamically assigns network settings, including IP addresses to local devices. Because DHCP renumbers the IP addresses when networks are combined, this can proactively prevent IP overlap as well as other issues, such as simplifying management and configuration. DHCP can eliminate the need for physical network IP addresses that your organization must manage with virtualized IP addresses. Because IP addresses are no longer static, your network will not have the same IPs as other connecting networks. You can also make IP addresses essentially permanent by assigning long lease times to the addresses.