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What you need to know about duplicate IP address detection

Read valuable expert advice about what causes IP address conflicts, and learn how to detect duplicate IP addresses in this tip.

What's the best way to discover a duplicate IP address in a network? If the network has a router, is it still possible to detect the duplicate?

Duplicate IP addresses can pose a challenge for network administrators. Fortunately, there are methods to detect duplicate IP addresses and resolve these issues.

First of all, let's discuss what typically causes IP address conflicts. IP address conflicts occur when two devices on a network are assigned the same IP address, resulting in one or both being disabled and losing connectivity until the conflict is resolved. IP address conflicts are often the result of configuration errors including: assignment of the same static IP address by a network administrator; assignment of a static IP address within the DHCP range (dynamic range) resulting in the same address being automatically assigned by the local DHCP server; an error in the DHCP server; or a system coming back online after an extended period in stand-by or hibernate mode with an IP address that has been reassigned and is in use on the network.

So, how can you detect if there are duplicate IP addresses on your network? Community member Sudhanshu recommends sending ARP packets to that IP address in order to detect the duplicate IT address. All machines with same IP will revert with the MAC address. If you are using a security solution, he recommends blocking those MAC addresses. If you are using tools to manage your assets, you can also use those to detect which users have the duplicate IPs. Nmap is a useful network scanning tool, as this Community member points out. You can use Nmap for an ARP scan on the local subnet.

Community member BlankReg explains that if you are using a Cisco router, you will find that the router sends an ARP for its own IP address when it connects to a network, or when the IP address is changed, so this will allow for detection of duplicate IP addresses.

According to information on Microsoft's support website, all Microsoft TCP/IP protocols currently shipping can detect duplicate IP addresses in most cases. Read more about using this method to detect duplicate IP addresses here.

Learn more on how to respond when your network has a duplicate IP address detected:

Note: This tip is a compilation of advice from various experts on our site.

This was first published in December 2013

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