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A single PC with one physical NIC, one virtual NIC (two IP addresses)

Posted on 2009-07-08
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I need to configure a PC to communicate mainly on network 10.11.0.0, however one piece of software on this PC (GE Fanuc Proficy iFix MBE [MODBUS over ethernet] driver) needs to communicate with a MODBUS gateway with address 10.12.17.17 on network 10.12.0.0.

All equipment shares the same physical network. The IP address of the MODBUS gateway (10.12.17.17) must stay the same.

I want to do this with a single physical NIC, and I can't simply add a second IP address to the NIC via the usual way (TCP/IP .. Properties .. Advanced) because the software seems to get confused when it's pointed to a NIC with two IP addresses.

This leaves me to think of other possibilities, such as:
* Can I install some kind of virtual NIC driver/application which operates through the physical NIC?
* Can I add some kind of static route to get around this?
* Perhaps software NAT could be configured on this PC?

Is there a neat way to do this, or should I just install a second physical NIC & patch it to the same switch?

Thanks.
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Question by:gpaeng
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TekServer earned 500 total points
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I suspect that whatever problem the software has with a second IP address on the current NIC will probably rear its ugly head again with whatever you try (virtual or physical NIC).  Any chance we can get more details on that to try to fix the underlying problem?

Hmm ... something has just occurred to me while looking back over your numbers.  I assume you're using a subnet mask of 255.255.0.0.  Are there any other subnets connected to this network, either physically or via VPN (or anything else)?  If so, what are their ranges?

I'm thinking we could solve this with a bit of supernetting - change the subnet mask to 255.248.0.0 (just on the machine in question, for now, and possibly the MODBUS gateway), and the subnet would then include the range 10.8.0.1 - 10.15.255.254 so that the MODBUS would then be on the same subnet with the PC.

Of course, there might be unintended consequences related to this ... we'd need to make sure this won't cause conflicts with the rest of the network first.  It's food for thought, though.

:)
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by:gpaeng
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TekServer,

Many thanks for your response. I'd never heard of supernetting before. I tested your suggestion in the office and it worked nicely. Both the PC and Modbus gateway needed the same mask (255.248.0.0).

More details on the MBE driver behavior: The driver seems to read the 'lowest' IP address that's configured on the network interface you nominate for it to use. Then, when you enter the IP address of the device you want to communicate with, it seems to check whether the device is on the same subnet, and if not, it doesn't work. The MBE driver does not seem to be aware of any IP addresses configured on the NIC besides the 'lowest' one.

(eg. the PC's NIC had IP addresses 10.11.9.33 & 10.12.0.121, so in this case the MBE driver was only willing to talk to devices on the same subnet as the 'lowest' IP address configured on the NIC (10.11.9.33)).

However, after speaking with our resident IT expert, we think we'd prefer to add a second physical NIC to avoid having unusual configurations such as this, and it's quite a complex and very important network with potentially severe consequences if we mess something up (it's a lead smelter).

Thanks again for your help.
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by:TekServer
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You're welcome!  Supernetting is not a new concept; you just don't see it very often any more.

That's basically what I expected on the MBE driver behavior.  My only concern with adding a second physical NIC would be that the MBE driver may still exhibit the same behavior, where it ignores the difference in the NIC's and still picks the first IP address.  Maybe the MBE driver can be bound to a particular NIC?  I'd say try it and see; no real risk there.

And speaking of risk, if the 2nd NIC option doesn't work out, there's really not much risk with the supernetting, especially if your network is well documented.  The only issues would come if you have additional subnets in your network, as I mentioned before, which could cause addressing or routing conflicts if the subnets overlap with the "supernet" (ex. if you already have a subnet 10.8.0.0/16).  Worst case scenario would probably be that the machine with the MBE driver would lose communication with something else on the network.

Anyway, let me know how it goes, and if you need anything further.
:)
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by:gpaeng
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Yes, the MBE driver can be bound to a specific NIC, which is why I'm very confident that adding a second NIC would work.

I'll let you know how it turns out.

Cheers!
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Thanks!

Glad I could help.

:)
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