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What does a Duplicate IP Address issue look like?

Within a subnet, it is important that only one station is assigned to an IP address. Otherwise, this can create problems for end-users as they try to utilize applications and network services. In this tech tip, we’ll explore how a duplicate IP can happen and what the symptoms are.

How it happens
In most cases, duplicate IP conflicts are due to configuration mistakes. Perhaps a technician added a device to the network with a statically set IP address that is also assigned to the DHCP address pool for that subnet. At first, the device might work just fine. But sooner or later, that same address could be assigned to a machine trying to join the network with a DHCP allocated address. Most operating systems are designed to detect and prevent this, however, some do not, which will cause the duplicate IP problem.

Another way a duplicate IP conflict could happen is that both stations are statically set to the same address. Good IP management should help to prevent this, as well as active network discovery and IP detection with an analysis tool like the EtherScope nXG prior to deploying a new device.

What does an IP conflict look like?
Imagine the headache that your mail carrier would have if another house popped up on your street with the same house address as your own. Which house is the “correct” one? You may get some of your mail, while others are directed to the other house and vice versa.

This is exactly what the two stations in conflict will experience when trying to access applications and services. Connectivity will suffer (or not work at all) because packets will not consistently be delivered to the correct device. ARP tables on all subnet devices will be constantly refreshed as the two endpoints compete for the IP address.

The EtherScope is designed to help quickly detect problems like this. Using ARP requests and analyzing the replies, it quickly detects and alerts you to duplicate IP conflicts, even pointing to the switch ports where the problem devices reside on the network.