Networking – Troubleshooting Strange IP Address from Router

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I've had a router set up for quite some time and it has been working perfectly. My wife called me to tell me the wireless internet wasn't working and when I came home I saw that my phone was connected to the wireless but there wasn't any internet, even though the wired devices had internet. My router is at 192.168.1.1 and gives out the normal IP addresses that you would expect 192.168.1.xxx. The IP address on my phone though was 10.55.142.7, still in the private network range, but totally nothing that should be assigned by my router. I reset the router, disable and enable dhcp, verified what IP address it should be assigning, but without success. I had another spare router that I wasn't using so I replaced the non-working router with the spare but all the wireless devices are still get the 10.55.142.xxx addresses and no internet connection. So I changed the SSID on my router to make sure someone else wasn't trying to used the same SSID but that didn't changed anything. There are no other router's in my house and nothing I can think of that would try to assign out IP addresses.

The wired connections to the router work just fine, but they all have static IP addresses. So I changed my computer to get an address through DHCP, and it got a 10.55.142.xxx address. If I changed the phone to have a static IP address it works! What is going on! It appears that DHCP isn't working but it can't be the router can it? The router was working and I replaced it with another one with the same results. I'm totally confused on this one.

Best Answer

It looks like you have a new DHCP server on your network.

The first thing you should try and see is which gateway you are assigned, when you are on the bogus subnet. If it is in the range 192.168.1.0/24, you may probably guess on which computer it resides, since you stated your pcs are all on static IPs.

Or, you may try turning off your pcs one by one, and see when your wireless devices, eventually, acquire an IP address in the 192.168.1.0/24 range. The last pc turned off is then the culprit.

Or, you may use a Ubuntu installation (drop an Ubuntu image onto a USB stick, and boot from that)to issue the following commands:

    sudo service network-manager stop
    dhcp -v eth0

from an ethernet-connected pc. The output, 

   dhclient -v
   Internet Systems Consortium DHCP Client 4.2.4
   Copyright 2004-2012 Internet Systems Consortium.
   All rights reserved.
   For info, please visit https://www.isc.org/software/dhcp/

   Listening on LPF/eth0/e8:e0:b7:be:72:6a
   Sending on   LPF/eth0/e8:e0:b7:be:72:6a
   Sending on   Socket/fallback
   DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 (xid=0x333a3d1d)
   DHCPREQUEST of 192.168.73.88 on eth0 to 255.255.255.255 port 67 (xid=0x333a3d1d)
   DHCPOFFER of 192.168.73.88 from 192.168.73.1
   DHCPACK of 192.168.73.88 from 192.168.73.1
   bound to 192.168.73.88 -- renewal in 35456 seconds.

shows this critical line, 

  DHCPOFFER of 192.168.73.88 from 192.168.73.1

which displays the IP address of the pc making the offer (192.168.73.1, in my case).

Or, assuming these IP addresses don't mean a thing (i.e., they fall in the 10.0.0.0/8 range), you may try installing a program called nmap, and run it with the command 

  nmap -T4 -A 10.55.142.1

where 10.55.142.1 is the gateway your pc is assigned, if it is not 10.55.142.1 please change it to its true value. This will tell you many things about the host pc, including (if you only have Windows machines) its name.

This should at least identify the pc where the new DHCP server is running.

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