Present 2 PCs with the same IP address

My company has a SCADA control system comprising of 2 industrial Windows PCs within a cabinet. These operate in a hot-standby configuration. One PC is designated as the online machine and this communicates with a number of remote PLCs etc. The other PC is designated as the standby and simple waits for the online PC to fail, which will then cause it to become the online PC. In addition, it is possible to manually force a PC switchover.

We have a situation where a Modbus client wishes to connect to our system. Ideally I would like to provide the vendor with a single IP address to connect to our system. However, I really need to provide 2 addresses, 1 for the online PC and another for the standby PC, which could be promoted to the online and thereby resume communications to the Modbus client. Which ever PC is online will server as the Modbus server.

I have been looking at ways of presenting our system as a single IP address. One potential option is Network Load Balancing using the features of Windows Server 2012. Ideally we would like to avoid introducing addition hardware into the cabinet. We have a Cisco L3 switch connecting PC1 and PC2 and this will also be used to connect the Modbus client.

What is the best way to do this?
Neil GowAsked:
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Paul MacDonaldDirector, Information SystemsCommented:
For your purposes, Microsoft NLB is probably a good solution.  It will let the two SCADA control systems keep their IP addresses, while you assign a third IP address to the NLB cluster for the Modbus server.
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aranaCommented:
hmm is there any reason you wouldnt want a script that changes ip's whenever it switches?
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Andy BartkiewiczNetwork AnalystCommented:
You have to use a load balancer, but it doesn't need to be in your cabinet. It only needs to able to reach the servers that you want to load balance
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Neil GowAuthor Commented:
Regarding the comment on scripts. I did have an idea of calling netsh in a script to change IP addresses of the NICs ie the IP address of PC1 is swapped with PC2 and vice versa. I have reservations of this though. We could encounter problems with IP conflicts etc.

Since I have 2 PCs, could the NLB be configured on both PCs or should the NLB be configured on a separate 3rd PC? Ideally, I want to be able to have PC1 with an IP address of 192.168.0.1 and if this PC is switched off or if I manually trigger a switch over, the PC2 becomes 192.168.0.1.
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Paul MacDonaldDirector, Information SystemsCommented:
"...could the NLB be configured on both PCs or should the NLB be configured on a separate 3rd PC?"
NLB would run on the two nodes.  Ideally, a third computer would act as a witness server.  You would end up with:
1) PC1 with whatever IP address it has now
2) PC2 with whatever IP address it has now
3) NLB Cluster which will have its own IP address and will always point to whichever machine owned the cluster at the moment.
4) Optional witness server, which just has a shared folder to help coordinate PC1 and PC2.
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