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Duplicate Name on Network with Internal and External IP address?

Posted on 2011-04-28
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Last Modified: 2012-05-11
Hi,

I have a PC with tow NICs one gets its IP from the DHCP server and the other has a static external IP address, when I power up the PC I get a duplicate name error.

The DHCP server is a 2wire BT business Hub and lists the two NICs with the same PC name.

is there any way around this error. the two NICs are on seperate networks but the both have to go through the 2wire router and I presume this is what generates the error.

Thanks
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Question by:compbuild
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Expert Comment

by:Cherukuri30
ID: 35484288
There are 2 possible reasons for this.
1. If you have a network then one of the pc is the network has the same name as the PC which has 2 NICs

2. Is your external IP Address belongs to your ISP? If yes then you might me under shared pool of bandwidth where other unknown user's PC has the same name who is also in the same shared bandwidth pool

If name of your computer is not a big concern then change the name of your computer.
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Author Comment

by:compbuild
ID: 35484460
There is no other computer on the LAN with the same name and I can see the name listed twice on the router with two different IPs one internal and one external, according to BT's technical support there is nothing they can do to stop this and reccomend I set up another DHCP server to seperate the LAN from the BT router.

Is there any way I can give a name to the NIC on the PC so it identifies itself on the router with a different name?
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vivigatt earned 2000 total points
ID: 35490416
AFAICT, Duplicate names errors are related to NetBIOS, aka "Windows network". In your case, it would be NetBT (NetBIOS over TCP/IP), unless you use NetBEUI or IPX (which would be odd!)

Some leads:
Can you put the 2 NICs in separate VLANs?
Then, the "broadcast NetBIOS name resolution" messages should not be sent from one NIC to the other, since your "router" would separate the VLANs.

If you do use a WINS server, make sure that only the "private NIC" is set to use it. The "public NIC" should not use it.

Another way around is to unbind "Client for Microsoft Windows" from the public NIC:
Open the network adapter properties for the public NIC and uncheck "Client for Microsoft Windows" and "File and printer sharing for Microsoft Networks" for this NIC. NOte that you might also achieve the same result by configuring your firewall so that it does not allows "Microsoft Windows Networking" on the "public NIC".

If nothing here works, please share some more details, such as the ip configurations of the NICs and router, the exact error message you get and where it is displayed...
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Expert Comment

by:pwindell
ID: 35505574
is there any way around this error. the two NICs are on seperate networks but the both have to go through the 2wire router and I presume this is what generates the error.

If they are on two different networks then how are they "going through" the same "2-wire device"?

If the "2-wire device" provides the means to do so,...then delete both of the DHCP Leases for that name and let it start over.   The Duplicate Name error is perfectly capable of being a false positive if the same name is listed twice in the DHCP Leases with a different IP on each one.
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Expert Comment

by:pwindell
ID: 35505611
Please be more accurate in your descriptions so that we are not chasing our tails in circles.  Saying things like "2wire BT business Hub" when there is no way in the world that it is a Hub,...and the other statement of,..."two NICs are on seperate networks but the both have to go through the 2wire router",....just don't make any clear sense and creates confusion.
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Author Comment

by:compbuild
ID: 35510949
Well I don't know what else to call it since it is a BT Business hub made by 2wire

http://business.bt.com/broadband-and-internet/internet-access/broadband/more-about-routers/

I know it is a router but this is what bt call it,also we have a range of static IP addresses from BT, the Business hub serves out bothe these and the internal LAN addresses, so therefore the two NICs are on seperate networks but the both have to go through the 2wire router,

I hope this clears up your confusion
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Expert Comment

by:vivigatt
ID: 35511385
If you want us to help, can you please try to answer the questions we asked ?

It seems weird to me that you have 2 different subnets on your "hub" (which is actually a switch + a router/NAT) without separating them with VLANs.

Please re-read the post that experts wrote and try to answer the questions or to make it clearer for us to be able to help.
My best guess would be that disabling "Client for Microsoft Windows" for the NIC that has the public address could solve your issue.

If not, details may help.
For instance exact error messages, on what host do they appear, IP configuration etc. might help.
A network trace (wireshark or MS netmon) may also help (make sur you record the trace when the duplicate computer name error message actually appears)
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Author Comment

by:compbuild
ID: 35511570
Apologies vivigatt, I was in a bit of a hurry earlier and just wanted to answer pwindell quickly, the error was appearing as a little pop up yellow triangle down in the system tray where the network connection icon is.

I have unticked

File and printer sharing for Microsoft Networks
Client for Microsoft Windows

And this has stopped the error showing up.

Thanks for your help

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Expert Comment

by:pwindell
ID: 35513046
vivigatt
It seems weird to me that you have 2 different subnets on your "hub" (which is actually a switch + a router/NAT) without separating them with VLANs.


Hi vivigatt,
I don't think there would be any VLANs because nothing is vitualized. It is all physical.  It has two layer3 intefaces just like any other retail NAT Box you can go buy.  The DHCP would be expected on the LAN side of the device,...but I seriously doubt it is providing DHCP on the External WAN interface.  It would not be impossible, but certainly not the "norm".

compbuild:
Anyway, my suggestions would be thus,...assuming this is not a home-user network or a business running their LAN like a home-user network:

1. Disable DHCP entirely on the NAT box
2. Run a real DHCP Server on the LAN
Problem solved

I think the root of the problem is that maybe the DHCP Service on the NAT Box, which may actually, and unfortunately, be serving both sides. has somehow registered a lease to the same machine twice with one being a LAN IP and one being an external IP,...this could happen if a LAN machine was mistakenly plugged into the WAN side of the device,..and then "whoops",...moved back to the LAN side where it should be,....yet the WAN side DHCP lease remained "lingering".  Deleting the Leases would solve the problem but I have no idea if the NAT Box is sophisticated enough to allow an Admin to individually delete a Lease entry.  In my own opinion a NAT Device has no business being the DHCP Server for a business or Corp LAN,  that job needs to fall on a real DHCP Server that has the intelligence to interact with Active Directory and allow DHCP, DNS and WINS to all interact properly together.

I have unticked
File and printer sharing for Microsoft Networks
Client for Microsoft Windows
And this has stopped the error showing up.


That is fine as a troubleshooting step. But that is in no way a solution, it is just a band-ade that hides the problem.  I can say with certainty that business networks across the world are not disabling those things to prevent duplicate machine name error.

If someone has dupe machine name errors they do:

1. If the error is true and not a "false alarm" then you find the two machines with Dupe Names and change the name on one of them.  Problem solved

2. If the errors are false alarms due to stale or "stuck" records in DNS, WINS, or DHCP,...then you go into those services and clean up the faulty records and let the service recreate the records correctly and automatically.  Problem solved.

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Author Comment

by:compbuild
ID: 35513346
Thanks for all your help so far, it is very useful

I have 5 static IP addresses provided by BT the BT hub can serve these out, when you connect a device it shows up in the device list and you can pick whether you want it to be assigned an address from the Private Pool (LAN) or the public pool (WAN) or you can use static IPs configured on the NICs. It does not matter which port on the hub you plug the device into, the fact is both NICs are plugged iinto a LAN switch in a different room from the BT hub

"1. If the error is true and not a "false alarm" then you find the two machines with Dupe Names and change the name on one of them.  Problem solved"

There are not two machines with the same name, the fact is there are two NICs in one machine lets call it workstation1, now the deive list on the BT hub seems to work off MAC addresses rather than device names so I have the situation where in the device list I have

Workstation1 DHCP enabled with an IP in the 192.168.1.x range

I also have

Workstation1 static IP with one of the 5 public Staic IP addresses eg 81.105.xx.xxx

So the BT Business hub see two devices (mac IDs) with the same name in this case Workstation1.

I hope this clarifies the situation
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Expert Comment

by:pwindell
ID: 35513411
You can't plug both nics of the device into the same LAN Switch,...you just don't,...ever,...never,....never ever.

The 5 addresses do not come from the BT thing.  They come from the provider.  The BT thing may be running DHCP and give you the ability to use it that way,...but it is not the authoritative source of the IP#.  You can use those Public IP#s for devices on the Public side of the network (when things are cabled correctly) even if you unplug the BT thing and throw it out in the street,...which is pretty much what I think you should do with the thing and replace it with a more normal device.

I've pretty much said everything I can say about this.  At this point I suspect that anything I say would be just repeating or rehashing what I have already said.
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Author Comment

by:compbuild
ID: 35514446
Point taken about them both being plugged into the same LAN switch, The set up did work correctly in that I could use the external address as an FTP site and I also had a cisco RV042 plugged in as a VPN device, again this worked as it should albeit a bit slowly.

I always though it was abit strange to have them all plugged into the LAN.

Any way I have reconfigured so that the BT hub is acting like an internet switch as such and the RV042 is the DHCP server for the LAN the second NIC on the workstation can now be plugged into the BT hub.

A much better set up all round, thanks for your input
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Expert Comment

by:pwindell
ID: 35514455
Ok.
Good luck with it.
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