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A duplicate name has been detected on the TCP network

Posted on 2011-03-25
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Last Modified: 2012-05-11
I am getting this error in my System event log.
A duplicate name has been detected on the TCP network.  The IP address of the machine that sent the message is in the data. Use nbtstat -n in a command window to see which name is in the Conflict state.

The 2003 Domain Controler system has 2 network cards in it. One is connected to the ISP's network and the other is connected to the local network. This is what I get when I type nbtstat -a command in at the DC. It returns the name of the just rebuilt windows 7 computer that will not join the network. I am left with the feeling that the error in the event log and the fact that the computer that wont join the network are related. I am not sure and if it is related, I would like to know how to fix the problem.

Thanks

C:\>nbtstat -a 192.168.1.99

Optimum:
Node IpAddress: [192.168.1.97] Scope Id: []

           NetBIOS Remote Machine Name Table

       Name               Type         Status
    ---------------------------------------------
    RUFFONESYS     <00>  UNIQUE      Registered
    WORKGROUP      <00>  GROUP       Registered
    RUFFONESYS     <20>  UNIQUE      Registered

    MAC Address = 00-18-F3-45-F3-B8


Local Area Connection 3:
Node IpAddress: [192.168.1.45] Scope Id: []

           NetBIOS Remote Machine Name Table

       Name               Type         Status
    ---------------------------------------------
    RUFFONESYS     <00>  UNIQUE      Registered
    WORKGROUP      <00>  GROUP       Registered
    RUFFONESYS     <20>  UNIQUE      Registered

    MAC Address = 00-18-F3-45-F3-B8

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
C:\>nbtstat -n

Optimum:
Node IpAddress: [192.168.1.97] Scope Id: []

                NetBIOS Local Name Table

       Name               Type         Status
    ---------------------------------------------
    VELVIT         <00>  UNIQUE      Registered
    PRUS          <1C>  GROUP       Registered
    PRUS          <00>  GROUP       Registered
    VELVIT         <20>  UNIQUE      Registered
    PRUS          <1B>  UNIQUE      Registered
    PRUS          <1E>  GROUP       Registered

Local Area Connection 3:
Node IpAddress: [192.168.1.45] Scope Id: []

                NetBIOS Local Name Table

       Name               Type         Status
    ---------------------------------------------
    VELVIT         <00>  UNIQUE      Registered
    PRUS          <1C>  GROUP       Registered
    PRUS          <00>  GROUP       Registered
    VELVIT         <20>  UNIQUE      Registered
    PRUS          <1B>  UNIQUE      Registered
    PRUS          <1E>  GROUP       Registered
    PRUS          <1D>  UNIQUE      Registered
    ..__MSBROWSE__.<01>  GROUP       Registered

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Question by:ruffone
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by:Justin Yeung
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it looks like you are running 2 network cards with same subnet.

you should have a different subnet running as the gateway for the local subnet.

both IP addresses will register in DNS reverse lookup zone and that where the conflict is coming from
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by:uanmi
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Justin is quite correct. You should change the internal subnet to 192.168.2.xxx or 10.xxx.xxx.xxx
this will also ensure that routing between the subnets works correctly.
regards, Mark
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by:ruffone
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Thanks for the response men! So when I go to the Internet Protocol property page of the adopter that connect the local network. It has an IP address: 192.168.1.45 and a Subnet mask: 255.255.255.128. The other adopter get it's address assigned automatically from the router which get it's address from the ISP. I change that subnet mask to 255.255.255.0 I am not sure if that will fix the problem. Beyond that there is 2 routers on the network with 2 DHCP zones setup. One issues IP from 192.168.1.1-127 and has a subnet mask of  255.255.255.0 the other one does IP from 192.168.1.129-256 and has a subnet mask of255.255.255.128. Those 2 may have being showing the conflict before. this is what they are now but I am still showing the error in the event log.

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by:uanmi
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Hi, what is the address currently showing on the WAN adapter?

On a LAN where you're using internal IP, I would recommend using separate subnets for zones and not try to use one subnet divided into two. I would need to see what is happening on the LAN, but it could be that one of the DHCP or routers has been changed and is now seeing the wrong areas or issuing DHCP to the wrong area.

If the WAN address is also 192.168.1.xxx then you have a problem that must be fixed. The LAN subnet should be changed to another subnet, say 192.168.2.xxx or 10.xxx.xxx.xxx

Routers are good for routing, so if you want to divide your LAN use different subnets for this. Then you would not have the possibility of DHCP or other errors.

regards, Mark
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by:ruffone
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Thanks again for responding. This is what I am doing. I have a connection to the ISP that has a public IP. That is connected to a router. The IP of the router is 192.168.1.1 With a subnet mask of 255.255.255.0. This router has a DHCP Zone assigned that issues 3 IP. One to a second router (192.168.1.100) and one to the Domain Controller that has 2 network cards (192.168.1.97) The second card on the DC is connected to a switch that the other computers on the network is plugged into. They get DNS service from the DC while the DC communicate with the ISP. Pretty much the same thing is on the second router. It all work for the most part except now trying to get this client that has being pulled off for maintenance back on there is a problem. Strangely though all the clients plugged into the switch seem to get there IPs from the router. All except for the second card in the DC shows up in the router. It looked off but it worked so I moved to other things. But that is a stat assignment so maybe it shouldn't show up on DHCP. As to the subnet assignments you suggested I am not sure how I would assign those. (192.168.2.xxx or 10.xxx.xxx.xxx)
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by:uanmi
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Hi, one problem is that your mixing subnet masks.

The dc should have the same mask on the WAN card as the router.

The dc should use a second subnet on the LAN card. 192.168.2.xxx

Another approach is to remove the second network card from the dc and remove the need for this server to act as a subnet router.

The point here is that dc is currently acting as a router for the LAN subnet and and this means the LAN pcs are getting to the WAN routers DHCP. The approach that you have taken normally will cause a routing table to be created on the dc where it will route traffic from the LAN to the WAN router. Is this what you want? If it is, the dc LAN must be set to another subnet 192.168.2.xxx with mask 255.255.255.0

The second router 192.168.1.100 what is it connected to?

regards, mark
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by:ruffone
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The second router is connected to the first router because the ISP hardware only has one port and I need some isolation for a second set of clients
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by:uanmi
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ok, I recommend you simplify your setup. It is not the standard approach now to use a DC server as a router and for it to have two NIC. This will work, but only if you use a separate subnet for each NIC.

Therefore I think you have two choices to prevent issues:
1. remove the dc second NIC - connect the LAN switch to the router port.
2. move the dc LAN to 192.168.2.xxx mask 255.255.255.0

regards, Mark
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by:ruffone
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>The dc should have the same mask on the WAN card as the router.
The WAN get its settings automaticly from the router

>2. move the dc LAN to 192.168.2.xxx mask 255.255.255.0
Does this mean that my clients will have to be assigned 192.168.2.2, 192.168.2.3...?
and does the DC have to be working all the time? I think this was the reson for the setup the way it was because if the DC was down there was not a big scramble. Clients could go hang out on the internet until it was back in service
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uanmi earned 500 total points
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Hi,

are you saying the dc is connected to router 1 through the WAN NIC and also to router 1 through the LAN switch with is connected to router 2 which is then connected to router 1? If so this is a circle and not advised.

The dc should be connected to a switch that also has all the pcs connected. This switch should be connected to the WAN router.

I do not fully understand what router 2 is connected to, but it should not be connected to the same switch as the dc and the LAN pcs.

If you have a second network of PCs then they could be connected to router 2 and router 2 connected directly to router 1. This would provide for a DMZ between router 2 and router 1 so as to isolate both networks.

regards, Mark
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by:m_walker
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It sounds like you almost have a loop in your network.  
If the DC is you DNS for your clients, then they cant surf if its down as they wont find the IP of the remote hosts.

As outlined above I think you need to flatten out your network a little.  The 2nd router does not seem to provide any value.

(some of the below has already been said)

I this was me, I would

Some Cleanup works...
- Leave the router that connects to the internet in place.
- Remove the 2nd router
- Shutdown/Remove 1 nic in the DC
- Connect the inside port on your ISP router to the switch
- Connect the DC Nic to the switch
- Connect all other computers/devices to the swith

IP Addresses
- Leave 192.168.1.1 255.255.255.0 on the router
- Assign 192.168.1.2 to the DC (Fixed/Static)
- either the Router OR DC for the dhcp server.  Set range to 192.168.1.11-192.168.1.254 (255.255.255.0)
Ensure the default gateway is 192.168.1.1 on all inside devices.
- DNS Server can be the DC Box, it would be good to have a backup dns server so they can surf if the DC goes down.

I have assumed that the ISP router is performing  NAT and some firewall functions.

Note: I dont want any points if you find this useful, as I have just collected what others have said and added a little more detail as I would do it... I am just tring to mak this a little clear for you.  Hope it helps.

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by:uanmi
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so yes, the LAN PCs should be 192.168.2.2, 192.168.2.3 and so on. The dc should be on the same range, say 192.168.2.100. DHCP is good and you should register the IP for each computer on the LAN using DHCP. To do this you need to note the MAC address for each pc and set up a DHCP permanent registration for each pc and the dc server.
I would then set a range for pcs that normally do not exist on the LAN to get an IP. Make sure they do not overlap IP with the permanent registrations.

regards, Mark
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by:ruffone
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uanmi,
The setup you suggest is the setup I had originally. Where I got confused was what IPs and Subnet got assigned where. Thanks for hanging in with me. I really appreciate
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by:uanmi
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ok, let me know how you go, nearly time for sleep here. I will be able to help tomorrow if you are still stuck.
regards, Mark
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by:ruffone
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uanmi,
Moving the DC to 192.168.2.xxx seem to have left me with a gateway problem. The DC connect but the client dont. I will double check the wiring but this is what I have.

DC
    IP        192.168.2.45
    Subnet    255.255.255.0
    Gateway   192.168.2.1
    DNS       192.168.2.45

CLIENT
     IP        192.168.2.99
    Subnet    255.255.255.0
    Gateway   192.168.2.1
    DNS       192.168.2.45

Thanks for all your help
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by:m_walker
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Ensure the DC Nic, the Router NIC and the Client NIC are all on the same switch.
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by:uanmi
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If you stick with two NIC in the server you would need to enable ras and routing - its the routing part you need. Read about setting up RRAS.

I recommend you connect the dc and the pcs to the switch and the switch to the WAN router. Disconnect the direct connection from the dc server to the WAN router. Turn router 2 off for now as it is surplus to your needs.

Now setup your DHCP to issue IP to each of the pcs and the DC server. I recommend you use DHCP static reservations for the dc server and the pcs - to do this you need to note the MAC address of the dc server and the pcs.

You only need one subnet with this simple star configuration. So you can use 192.168.1.xxx and mask 255.255.255.0

if the IP of the router1 is 192.168.1.100 this would be the gateway address that you set in the DHCP.

regards
Mark
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by:uanmi
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to get the mac address do a ipconfig /all on each machine. Note the MAC address and then use this to create DHCP static reservations.

This is the best way to control pcs / servers on the network.

regards
Mark
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by:ruffone
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Thanks a lot for hanging in with me and helping work this through. The connection is much faster than before. the resources just pop right up though a bit unreliably. Just now I log in in and had to unplug the switch from the router before I got an IP. Maybe I should shut down DHCP on the router. But my event log is now good and that issue is gone. Looking back at the logs this thing being running since 2005. This is a good pick me up.

Thanks
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