At my prior employer whenever we had two systems with the same IP address - particularly if they were windows systems we'd get a window popup on our system stating: "Windows has detected an IP conflict".

At my new employer, we have had IP conflicts and that message doesn't come up. What is different that is causing windows to behave differently? Is there some feature on our network switches which need to be enabled or maybe something at the firewall/gateway level?

I should state in both networks IP addresses are statically assigned - no DHCP is used.

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    Switches don't know or care about ip addresses. How do you know there are undetected ip address conflicts? What's leading you to believe that? What are you seeing or experiencing? – joeqwerty Feb 27 '16 at 15:08
  • Because I unintentionally setup two VMs with the same IP which caused me about a day of grief trying to track it down :) The IP conflict still exists.... but there were no warnings on either system. – Brad Feb 27 '16 at 18:32
  • Here's the dialog I'm referring to: i.stack.imgur.com/KPHGK.png Why am I not getting that? – Brad Feb 28 '16 at 0:55
  • @joeqwerty Your comment is correct if by "switch" you mean a device that operates only as a layer 2 switching device. But he may well have a box that acts as a switch that he calls "the switch" that cares very much about IP addresses. – David Schwartz Feb 28 '16 at 5:30
  • @DavidSchwartz: Yes. I was referring to a Layer 2 switch in the strict sense. – joeqwerty Feb 28 '16 at 5:36

If you're managing the adresses via static assignment conflicts may occurs.

From the switches point of view it's hard to distinguish when you're talking about a conflict. If you've clusters implemented that have a shared address which can be failed over between nodes, at some point of time it's expected to have an address conflict (until announcements are received on the whole network).

DHCP is the solution for such problems (even if under specific circumstances conflicts may also arise here). Depending on the network size i strongly recomment to think about using DHCP (which also gives you a bunch of additional options that you can handle easily then).


It seems there's confusion about the benefits of using DHCP. Using DHCP in your network (or even multiple networks when setup on firewalls via relays and so on) you have a central point for key network configuration that provides addresses, gateways and additional runtime configuration like DNS, NTP and much other services.

So, DHCP can help you to organize the usage and assignment of addresses in your networks quite well. It by far doesn't replace any documentation - which in current state [as there are conflicts] seems to be missing.

  • This is in a datacenter environment where addresses cannot under any circumstances ever change. Otherwise things like load balancing break. I could setup reservations for each of my servers but if I have to do that for every server - I might as well just use static addresses anyway. As I've said I've worked in environments where windows did popup an alert when there was an ip conflict - I just don't understand what was different about that environment vs this one. If the switches don't help with that - maybe its something being done by the gateway router? I'm not sure – Brad Feb 27 '16 at 18:34
  • I found a linux tool called arpwatch which seems to watch arp traffic for multiple MACs sharing the same IP - it will also report new workstations it finds on the network. So that would be an alternative way of dealing with this - BUT it only listens on one network. We have five different network segments so I'd need 5 different linux boxes setup with arpwatch which seems like a poor solution to the problem. – Brad Feb 27 '16 at 18:36
  • Perhaps the vendor specific drivers were doing that? Although that seems unlikely. This Microsoft support article seems to suggest that all Microsoft OS's should be able to do this: support.microsoft.com/en-us/kb/120599 but clearly its not? I'm not sure why. – Brad Feb 27 '16 at 18:40
  • @Brad, you can still use DHCP in such an environment. Use reservations so that each server gets the same IP address every time. You then have a central location from which to manage your IP addresses, and it can help to eliminate IP address conflicts. – Ron Maupin Feb 27 '16 at 19:34
  • Seems like if you have to use reservations for the majority of the systems managing DHCP is just a lot of overhead. Regardless this doesn't answer the original question which is - if there's an IP conflict why isn't windows displaying a warning :-) – Brad Feb 28 '16 at 0:43

Check RFC5227 and the KB below. It explain more the process. As you can see it use ARP packet, which can be blocked at the switch level / local firewall level.

  1. Subsequent passive detection that another host on the network is inadvertently using the same address. Even if all hosts observe precautions to avoid using an address that is already in use, conflicts can still occur if two hosts are out of communication at the time of initial interface configuration. This could occur with wireless network interfaces if the hosts are temporarily out of range, or with Ethernet interfaces if the link between two Ethernet hubs is not functioning at the time of address configuration. A well-designed host will handle not only conflicts detected during interface configuration, but also conflicts detected later, for the entire duration of the time that the host is using the address (Section 2.4.).

and most of all, check that kb:https://support.microsoft.com/en-us/kb/120599

At system startup, when the IP protocol initializes, it sends an ARP request containing its own MAC and IP address so that other computers can update their ARP caches. If there is already a computer using the IP address, the "older" computer will respond with an ARP reply containing its MAC and IP address, indicating a conflict. Unfortunately, many other computers may have already updated their ARP caches with the new mapping. At that point, the "younger" computer that is initializing needs to do two things:

Repair the ARP cache on all affected computers. Cease using the duplicate address. Computers running Microsoft TCP/IP will send out a new ARP broadcast to re- map the ARP cache on all affected computers. This new ARP will contain the MAC address and IP address of the older owner of the IP address. After sending this ARP, the IP protocol on the younger machine will report the problem to the user and the stack will shut down. The stack should not be re-started until a unique address is obtained. Note that the computer may still function at this point if another protocol such as NetBEUI is loaded.

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