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Multiple unmanaged DHCP servers on network RRS feed

  • Question

  • What would happen if, for instance, I had a router (Router A) hooked into a modem and another router (Router B) wired into router A. Assuming both routers are acting as DHCP servers and there has been no special configuration to them (such as subnetting), how can it be that they never accidentally assign the same IP address to two different devices?

    I setup an experiment based on the above scenario and never had an instance of multiple devices accidentally getting assigned the same IP address. I don't understand why though. If they are able to coordinate the effort of assigning IP addresses, why is a device connected to router A not able to ping a device on router B? I receive a destination unreachable error, meaning it couldn't find a path to the device, but a ping from a device on router B to a device on router A works just fine. Could someone explain the reason behind this as well?

    I've had many introductions to networking before but I'm just starting to get down into the weeds. I will be getting my Network+ certificate within the next quarter of the year and I want to understand everything down to a science.

    Thursday, July 4, 2019 2:43 AM

All replies

  • Hi,

    DHCP is first-come, first-serve. You should not have two competing DHCP servers on the same network without some form of failover between them, otherwise you run the risk of having duplicate IP's on the same network.

    For your reference:

    https://serverfault.com/questions/368512/can-i-have-multiple-dhcp-servers-on-one-network

    I receive a destination unreachable error, meaning it couldn't find a path to the device, but a ping from a device on router B to a device on router A works just fine. 

    Did you use the same devices on router A and B. I think there is a problem of the routing between two devices.

    Firstly, You can ping router A, then ping router B, at last ping the device on router B.

    Best regards,

    Travis


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    Thursday, July 4, 2019 6:07 AM
    Moderator
  • Weirdly enough, I just checked again this morning and for some reason router B is giving IP's in the 10.0.0 range now whereas router A is still giving out 192.168.0's. 

    I'm not able to ping router B either, even though it's directly wired to  router A. Shouldn't router B be given an IP in the 192.168.0 range since it's connected to router A? And if this is the case, if I do ipconfig on a computer that is connected to router B, shouldn't it say that the default gateway is that 192.168.0 address, even if it is assigned an IP of 10.0.0.x?

    Thursday, July 4, 2019 1:06 PM
  • Hi,

    The assigned IP address depends on DHCP scope on the router. The DHCP scope on router A should be 192.168.0.0. 

    How did you connect router A and router B? How did you configure the LAN and WAN interfaces?

    Please check the IP of the interfaces and the routing table on the routers.

    Best regards,

    Travis


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    Friday, July 5, 2019 5:58 AM
    Moderator
  • I just not configured router B to bridged mode and turned off its DHCP server. I can now see all the printers that are behind router B but I'm still not able to contact the computer behind router B.

    For my own knowledge, why was it that I wasn't able to communicate with devices behind router B before? Is router A not able to communicate with devices behind router B? If so, why were devices behind router B able to speak with devices behind router A?

    Sunday, July 7, 2019 10:55 PM
  • Hi,

    It is mostly because router B has a route to router A, but router A does not.

    As a result, I suggest you check the routing tables in routers A and B. You can also add a static route if necessary.

    Best regards,

    Travis


    Please remember to mark the replies as an answers if they help.
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    Monday, July 8, 2019 6:19 AM
    Moderator
  • Hi,

    Just checking in to see if the information provided was helpful.

    Please let us know if you would like further assistance.

    Best Regards,

    Travis


    Please remember to mark the replies as an answers if they help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com

    Friday, July 12, 2019 6:17 AM
    Moderator