DHCP Issue

Posted on 2004-03-23
Last Modified: 2010-04-11
I am facing a problem with regards to DHCP. I have the following setup. Cable/DSL modem, Linksys 8 Port router, LAN and windows 2000 server. When I activate the DHCP server i get an error mesg that there is a conflict in the network regarding the addresses. I have stopped the DHCP service on the router have forced it to take a static address instead of dynamic addressing for the WAN connection type. I have a DNS server on the same unit. could someone tell me how i could creat a connection to the internet from my intranet and how i could solve this overlapping of ip address outter. I also get DNS refussed error could this be associated with ip configuration issues?

Question by:wilkersons
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LVL 27

Expert Comment

ID: 10661712
Yes, your DNS problem could be related.  Let me get this straight - you plug in your Linksys and the OUTSIDE interface is the one with a duplicate IP address!?!

Try turning everything off.  Turn the cable modem on first, wait 2 minutes.  Then turn the Linksys on and wait 2 minutes.  Then plug in ONE PC into the Linksys and test.  See if that works.

The 2K box isn't doing DHCP Server as well is it?
LVL 21

Accepted Solution

wyliecoyoteuk earned 500 total points
ID: 10661753
You seem a little confused, but I think that this will help.
The setup of the WAN DHCP client on the router is not the same as the DHCP server.
The router uses a DHCP client or a fixed IP addres on the WAN side.
It can also run a DHCP server for your network on the LAN side(this is often the default)
If you then run a DHCP server on your win2K server, there will be problems.

The simplest setup is DHCP server on the router, gives out DNS and Gateway to clients.
If you want to run a DHCP server on the 2K server, disable DHCP server on the router, give it a fixed internal address, and in the DHCP client options settings on the 2k box, add the router as Gateway and DNS server (secondary DNS server if you have an internal DNS server already).

Example (based on actual network)

Router WAN address from provider`s DHCP. internal address: netmask running DHCP, range upwards. (use the unallocated range up to 100 for fixed devices like servers or printers.)
The clients will get addresses in the range, netmask Gateway, DNS servers DNS server),

Of course without knowing more about your network size, configuration and useage, I cannot be more specific.

LVL 21

Expert Comment

ID: 10661836
plus. as psuedocyber spotted ( and I didn`t) if it is the outside IP that is the problem, unless you have been allocated a fixed IP by your provider (usually costs extra), you will have to use an automatically provided one from the cable modem.

Author Comment

ID: 10662136
Sorry people I should have been more specific. Yes your right wyliecoyoteuk the router WAN DHCP has notin to do with the 2k server DHCP. The issue is that even when i disable the Router DHCP facing the LAN and enable the 2k server DHCP i keep getting teh error "specified ranges either overlap or are invalid" not to mention DNS operation refused

"""""Router WAN address from provider`s DHCP. internal address: netmask running DHCP, range upwards. (use the unallocated range up to 100 for fixed devices like servers or printers.)
The clients will get addresses in the range, netmask Gateway, DNS servers DNS server),"""""

I have the following:: ROUTER

Router WAN address : DHCP
Internal Address:
Range: to

CLIENTS (Server)

Clients address: to
Server address:
default gateway:
Server has a DNS install on it address:
                          alternate address:


LVL 21

Expert Comment

ID: 10662743
All looks ok, but sounds like your DHCP scopes or DNS zones are set up incorrectly.
It is possible that you have a mix of ip addresses issued by both the router and the 2k dhcp server, thus when the 2k server tries to issue an address, there is a conflict with an address already on the network.
How many clients do you have? could you try setting the 2k range to -250? (you could always move it back after all the clients have moved over, how long this takes would depend on your lease settings)


Expert Comment

ID: 10662944
Try this first.

Turn off your Win2K machine.
Then do a hard reset of your router to factory specifications.
Make sure your desktop is set up for DHCP
Make sure your desktop does not point to your 2000 server for DNS
Then plug in your router and restart your computer and see if that works.

Once you have that working you can change the settings on your windows 2000 server. I reccomend using the IP range for your server and leaving the range for your DHCP.

Hope this helps
LVL 27

Expert Comment

ID: 10666979
I know you've disabled DHCP services on the router, but just for grins, why not change the scope to something silly, like say .5-.10.?  Then you would know the scopes don't overlap - even though one of them is supposedly inactive.

Any chance you have a protocol analyzer (like ethereal) and could look at the DHCP traffic?  Just to see who is requesting and who is replying?
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LVL 21

Expert Comment

ID: 10667592

yes, ethereal is excellent for this sort of thing :)

download it free from

Expert Comment

ID: 10673715
Simple, change your internal addressing to and up.  You need to differentiate your subnets.

Usually routers dont like to have both networks, inside and out, as the same subnets.  Try it and let us know.

Author Comment

ID: 10681626
blackss do u mean the internal address from the router or the address form the dhcp service/server ?
LVL 27

Expert Comment

ID: 10700234
Wilkersons, I think Blackss was thinking your WAN network and your internal LAN network were the same - but I didn't get that from your posts.

Were you able to try changing the DHCP scope on your router as I had suggested?

Is this still an issue for you?

Author Comment

ID: 10703815
Guys I will work on ur suggestions and ping u the results
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Expert Comment

ID: 10703833
Sounds good. :)
LVL 27

Expert Comment

ID: 10712802
How'd it work out Wilkersons?

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