Tackling IP Conflicts – Work Smart and Minimize Network Downtime

IP address conflicts are usually temporary, but you can’t always expect them to resolve themselves. In my previous blog, we looked at the various causes of IP conflicts and the difficulties administrators face when determining the source of a network issue and whether it’s actually an IP conflict. In this post, I would like to peruse troubleshooting IP conflicts and the fastest methods of resolution to minimize network downtime.

So when you see the blatant message staring at you from the screen, “There is an IP address conflict with another system on the network.” Network administrators would typically want to know, as quickly as possible, what system owns that address and where is it located? A relatively easy way is to find the MAC address of an IP address within the same network or subnet, ping the IP address, and then immediately inspect the local ARP table on the router. If you use a Windows PC, the following steps will guide you through this search:

  • Click on Windows ‘ Start’ , type ‘cmd,’ and ‘Enter’ to open the command prompt
  • At the command prompt, ping the reachability of the IP address that you want to locate
    • For example, ping xxx.xxx.xx.xx. If the ping is successful, you should see a reply from the remote deviceif the ping request doesn’t locate the host then you won’t be able to proceed with the next step
  • Now, at the command prompt type arp –a. The command should return a table listing all IP addresses your PC is able to contact. Within this table you can locate the IP address you’re looking for, then the corresponding column will show you the MAC address

This method for finding the MAC address with ‘ping’ and ‘arp’ typically works. However, if it does not, then you will have to take more time and effort to locate the offending MAC address. If you do not find what you are looking for on the first attempt, you will need to repeat this process on all routers until you find the offending IP and MAC address. Once you are successful in locating the MAC address, you need to find the switch and switch port that the offending IP address/device is connected to. Knowing this will help you to disconnect the device from the network. The following steps help locate the MAC addresses connected to a switch.

  • Issue this command on each switch in your network- ‘show mac-address-table’ (this is for Cisco IOS or compatible switches).
  • The command returns a list of MAC addresses associated with each active switch port. Check if this table contains the MAC address that you are looking for.
  • If you find the MAC address, then immediately consider creating new ACL rules or temporarily blocking the MAC address. In critical cases, you might want to shut down the switch port and physically disconnect the offending device from the network.
  • If you do not find the MAC address, repeat the command on the next switch till you find your device.

While these procedures help you locate a device on the network, they can be very time-consuming, require technical expertise and login access to network switches and routers.

There are two factors that complicate the effort of locating a device on the network. The first one is network complexity and the other directly relates to historical data availability. The above technique heavily relies on ARP caches. Unfortunately, these caches are cleared from time to time. If this data is not available, it is impossible to determine the location of a system. During a crisis, you would want a system that can help you locate issues fast and easy. Being alerted about an IP conflict before users start complaining or a critical application going down, is important to network reliability. To be able to quickly search for a device with its IP address or MAC address and locating it on the network reduces the time and effort involved in troubleshooting and eliminating issues caused by IP conflicts.

Today many IT solutions are offered that aid in effective monitoring and resolution of problems like IP Conflicts. These methods are much faster than manually searching for offending devices. Solutions such as these should offer the ability to:

  • Constantly monitor the network for IP Conflicts by setting up alert mechanisms
  • Quickly search, identify, and verify details of the offending device
  • Locate the offending device and immediately issue remediation measures to prevent further problems

So, what method do you find to be the most effective for troubleshooting IP conflicts? If it’s an automated solution, which do you use?

  • Typically, it has been my experience that a stale computer boots up and connects as being the root cause for most of our IP address conflicts.  Performing a reboot on one of the conflicted machines seems to resolve the issue.  Good communication between groups (help desk, servers, etc) is essential to light up the environment and perform the reboot.

    Never actually thought about automated tools to assist in resolving these situations.