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I have a windows domain with DNS that forwards to my ISP and DHCP that passes out addresses in the 10.0.0.X range.

Things were fine until recently, now when I reboot I get a 192.168.40.X address with an invalid DNS server so I can't access any network resources or the internet.

When I manually change the ip to a 10.0.0.X adress and then back to obtain an address and dns server it can talk to the DHCP server and pulls a good address along with a DNS server and default gateway and everything works for a while.

Also, it randomly reverts back to the private range, but i can do an ipconfig /renew to fix it when this happens. I'm assuming because the 10.0.0.x address is cached.

What would cause it to not pull a address from my DHCP server when I boot, and why does it keep reverting to the 192.168.x range?

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I agree with @heavyd you probably have a rogue DHCP server on that same VLAN in your network. I would set up DHCP snooping, if you have Cisco, and try to find him or her. If not ping the default gateway that was assigned when you got the 192.168 network and look at the ARP cache then go around to your switches looking for the MAC address that you saw in your ARP cache. Most likely the default gateway is the person that has set up the DHCP server, not a sure thing though. – matak Nov 15 '12 at 0:26

2 Answers

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You probably have another DHCP server running on the network somewhere. Check the configurations of your router and DSL/Cable modem. Any one of those devices may have a built in DHCP server that is responding to your DHCP request before the DC can respond.

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ipconfig /all|findstr -i -c:"dhcp server"

Once you have a machine with one of the bogus IP's, you find the DHCP server that gave it to you.

Breakdown:

ipconfig /all gives all the info about network adapters

| is a pipe. This takes output from first command and sends it to the next for processing.

findstr command looks for and returns entire lines that contain criteria

-i ignore upper/lower case

-c:"dhcp server" literal search. enclose in double quotes for criteria with spaces.

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