How to Find and Fix IP Address Conflicts

IP address conflicts are an example of textbook “network problems”. There are multiple causes for IP conflicts, and, to make things even more fun, the behavior of devices experiencing an IP conflict can vary. Let’s explore IP conflicts in depth to help better understand what they are, why they occur, and how to fix IP address conflicts.

What is an IP address conflict?

An IP address conflict is a common network issue that occurs when two or more devices on the same network have the same IP. Exactly what will happen when an IP address conflict occurs depends on the specific devices in place and your network configuration. Generally speaking, when multiple devices on the same Layer 2 network respond to the same IP, connectivity to those devices becomes unpredictable.

To help get an idea of what can go wrong when IP conflicts occur, let’s start with a simple example. Say you have three devices on an IPv4 LAN with a /24 (255.255.255.0) subnet:

• Device one: Your PC with the IP address 192.168.1.100
• Device two: A printer with the IP address 192.168.1.200
• Device three: A networked camera with the IP address 192.168.1.200

For the sake of our example, let’s say both the printer and networked camera are running a web server on TCP (Transmission Control Protocol) port 80.

You open your browser and attempt to access 192.168.1.200. The device you see will be whichever device most recently responded to your PC’s ARP (address resolution protocol) requests with its MAC (media access control) address. If you wanted to access the printer, and the printer is in your ARP table, you might not notice any problems. However, MAC entries only stay in your ARP cache for a short amount of time, usually a minute or two. That means a refresh of the page may wind up surprising you with the networked camera’s login screen. It becomes a race between which device can respond to an ARP request first, leading to undesired results.

As you can imagine, things only get worse if there are more than two devices responding to the same IP address. Long story short, IP address conflicts can make the affected devices unstable to the point of being unusable. And if these IP address conflicts occur on the IPs assigned to critical network devices, like a firewall or a router, your whole network can grind to a halt.

Sidenote: IPv4 and IPv6 address conflict differences

As we can see from the example above, understanding ARP allows you to understand the technical side of IPv4 address conflicts. If you’re not already familiar with how ARP works, check out “What is an ARP Table?” to get up to speed. In simple terms, IPv4 address conflicts boil down to If multiple MAC addresses respond to the same IPv4 address, ARP tables get unstable and connectivity to those devices becomes erratic.

But with IPv6, there is no ARP. So how do IPv6 MAC to IP address mappings get sorted out? With NDP (Neighbor Discovery Protocol).

NDP defines five types of ICMPv6 (Internet Message Protocol version 6) messages to enable functionality similar to IPv4’s ARP. For MAC address discovery, NS (neighbor solicitation) messages (which are ICMPv6 type 135) are used.

So, while there is no ARP involved with IPv6, the fundamental IP address conflict problem is the same as with IPv4. In both cases, MAC and IP addresses no longer have a 1-to-1 mapping and connectivity becomes erratic and unreliable.

What causes an IP conflict?

Now that we know what an IP address conflict is, let’s look at some common causes of IP address conflicts.

• Misconfiguration of static IP addresses. In many cases, an administrator may simply give a device the “wrong” static IP address. For example, an admin could typo an IP address or accidentally assign a static address that falls within a DHCP (Dynamic Host Configuration Protocol) server’s scope. Similarly, in networks with BYOD (bring your own device) policies, a user may bring a device that is configured with a static IP address that conflicts with an existing network device.
• DHCP server issues. Misconfiguration of DHCP servers, unauthorized or rogue DHCP servers on a network, or even DHCP server bugs can lead to IP address conflicts.
• Conflicting default IP addresses. Many devices — particularly embedded systems and IoT (Internet of Things) devices — come with default IP addresses where they can be configured. This can be great if you’re setting up one device… but if there are several identical devices on the same network, you can run into IP address conflicts.
• VPN configurations. Remote access VPN (virtual private network) connections are a very common way to enable work from home. If a VPN network’s address range conflicts with a home user’s network address range, IP address conflicts can occur. For this reason, it’s a good idea to avoid IP ranges like 192.168.0.0/24 and 192.168.1.0/24 for remote access VPN networks.
• Malware & malicious users. In addition to bugs and honest mistakes, some IP address conflicts can be a result of malicious intent. For example, ARP cache poisoning and rogue DHCP servers can be used by attackers in an attempt to compromise a network or steal data.

Types of IP conflicts

Given the common causes of IP address conflicts, we can group them into a few specific types. Understanding which type you’re dealing with can be useful when deciding how to resolve IP address conflicts. Broadly speaking, we can categorize the different types of IP conflicts as:

• IP address conflicts related to static/manual IP configuration. Things like the configuration of incorrect static IPs, conflicting default IP addresses, BYOD devices, and —on a slightly larger scale— VPN and home network address ranges, can fall into this category. In these cases, you’ll need to do some reconfiguration of the specific devices responsible for the conflict. More on this below.
• IP address conflicts related to DHCP servers. These types of IP conflicts include things like conflicting DHCP reservations, overlapping DHCP scopes, and DHCP server misconfigurations.
• IP address conflicts related to malicious actors. When you detect IP address conflicts due to malicious actors or malware, you have an even bigger problem on your hands. Before starting the cleanup on the networking side of things, you’ll also want to alert your security team (if you have one), and scan and remove any remaining security threats on the network.

How to resolve top IP conflicts

Because IP address conflicts can occur in different ways and on a wide variety of systems, there is no one-size-fits-all method to resolve them. That said, there are some tried and true methods for troubleshooting and resolving many of the top IP address conflicts, so let’s take a look at those. As we go, keep in mind that you may need to mix and match some of these steps to address real-world problems.

IP address conflicts with static IP configuration

These two steps apply when devices have been configured with static or manual IP addresses that conflict with other devices on your network.

1. Isolate the devices experiencing the conflict. The key step here is to make sure you can reliably get into one of the devices to reconfigure them. That might be achieved by turning off a switch port connected to one of the devices, accessing a local terminal, or simply attempting to reconfigure whichever device is currently in your ARP cache.
2. Assign a non-conflicting address. Once you’re able to reliably access one of the devices, give it a “good” IP address. Then repeat for other affected devices. What exactly a “good” IP address will vary based on the specific situation. In some cases, you’ll want to configure the devices for dynamic IP address assignment from a DHCP server. In others, you’ll need to manually configure a non-conflicting static IP that is outside your DHCP scope.

IP address conflicts with DHCP servers

While support for features like IPv4 address conflict detection, which is defined in RFC5227, can help DHCP servers prevent IP conflicts before they happen, IP conflicts can still occur. In practice, several types of conflicts can arise due to DHCP server bugs and misconfigurations. If your issue is related to a DHCP server bug, you’ll need a patch or workaround to address it. Beyond that, here are a few common fixes to try:

• Correct conflicting DHCP reservations. If your DHCP reservations conflict across DHCP servers or with static IP assignments, you’ll need to correct them on the server (see above).
• Reconfigure overlapping DHCP server scopes. If you have multiple DHCP servers on a network and their scopes overlap, you can run into issues with overlapping scopes. To resolve this, try re-scoping. For example, as opposed to two servers on a 192.168.1.0/24 network both configured with a scope of 192.168.1.50-192.168.1.100, configure one with a scope of 192.168.1.50-75 and the other with 192.168.1.76-192.168.1.100.

Remember that completely resolving these DHCP-related IP address conflicts will often entail making a correction on the DHCP server side, and possibly taking action on the affected devices (e.g. running “ipconfig /release” and then “ipconfig /renew” on a Windows machine).

How to detect, prevent, and mitigate IP address conflicts

Like many things in networking, with IP address conflicts an ounce of prevention is worth a pound of cure. In some cases, planning and policies can help prevent IP conflicts before they happen. In others, network devices and protocols can help.

Network monitoring and troubleshooting and network management software can help you quickly detect and resolve IP address conflicts early on. You’ll reduce downtime and confusion in the event an IP address conflict occurs.

From a planning perspective, proper IP address allocation and network design can go a long way. Getting your DHCP scopes and reservations right and preferring DHCP to static assignment for clients (as much as practical) can go a long way. Similarly, avoiding common “home” network address ranges, like 192.168.1.0/24, can help you avoid IP address conflicts.

Switches and DHCP servers also offer methods to help address ARP cache poisoning and other causes of IP address conflicts. If your switches support it, you may want to look at enabling DHCP snooping to prevent rogue DHCP servers. Cisco’s IP Source Guard and Dynamic ARP inspection are two popular features that can help address IP address conflicts.

Further, one of the advantages of using IPv6 instead of IPv4 is the wealth of addresses which — at least statistically — lowers the chances of an IP address conflict. Additionally, IPv6 neighbor solicitation messages provide a method to automatically detect conflicts on the same network segment. Simply put: IPv6 has a decent amount of upside.

Finally, effective network configuration management helps isolate when problems began, enabling you to roll back to a “known working” configuration. With network configuration backup tools you can reduce your MTTR (mean time to recovery) if an IP address conflict arises due to a misconfiguration.