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I'm helping a friend to set up a second router, but we ran in some strange problem. Here is the topology of the network and related information.

What did we do: connected one of R2 LAN port to one of the R1 LAN ports, then we set R2 LAN IP address to be in R1 subnet, then we disabled DHCP server on R2.

After doing that I assumed that users in Room 3 would get the IP assigned by R1, but that is not the case. PC1 got IP assigned from DHCP server it just don't seem to be from R1, and it's suspiciously not a private IP address.

Adding to that, neither PC1 after changing it's subnet to 192.168.1.0/24 or R2 can ping R1 LAN address: 192.168.1.254, but PC1 can access internet.

Can anyone tell me what is going on? And any suggestion on how to debug this and get it fixed would be wonderful.

PS. Web console model used for configuring R2 can be found here: http://uk.tp-link.com/resources/simulator/TL-WR841ND_V10/Index.htm

Update: We determined that DHCP responses are coming from "internet" most likely ISP, because when the cable to the internet was removed from R1 (same topology) the DCHP request from PC1 (connected to R2) was unsuccessful. Still in the search for the solution.

Update: We came to a solution. The problem was that originally ISP had configured R1 and made different configurations for different ports on R1 not telling that they differ, we found that out while trying @some user suggestion. So basically we where one port away from the solution.

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Base on IP address lookup,

http://whatismyipaddress.com/ip/62.84.41.51

the IP belongs to "ALMANET broadband clients in Astana". Does it ring a bell?

I suspect there is another DHCP server in your network and the PC1 maybe at some point being set as DMZ. I would suggest a few steps to troubleshoot.

  1. Unplug the R1-R2 LAN cable from R2 and connect it directly to the PC1. Does it still get the same 62.x.x.x IP?

If yes, the DHCP response is coming from the your R1 or another device from R1.

If not, you should focus on the devices connected to R2.

(You also just unplug the R1-R2 LAN cable, restart PC1 and see if PC1 gets an IP at all, in which case, the yes/no actions are reversed)

Keep the setup above and take devices offline one by one until you isolate the device that is giving DHCP.

If in the end, you determined that it is R1 assigning the IP. You can also change the MAC address on your PC1 and see if the behavior changes.

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  • Hey, thanks for suggestion, I just updated the question.
    – Chakijs Terzētājs
    Commented Jul 13, 2017 at 6:17
  • base on your update, if you plug PC1 directly into R1, your PC1 should get the same public IP. Were you seeing that? If you are not familiar with changing MAC on PC1, you can check if your R2 supports MAC cloning. You can clone PC1's MAC to R2 and move R1->R2 cable from R2's LAN to WAN port. If issue was due to the MAC address, R2 should get the same public IP.
    – some user
    Commented Jul 13, 2017 at 6:42

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