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I have a question about IP conflicts. Let's say you have a company building full of PCs on a network. Then a firm buys out that company and institutes its new system and new network for that building. But the old PC IP's are in conflict with the new system. How would you re-IP (is this the correct terminology) the servers and PC to make them work?

JoeB
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There is no one-size-fits-all answer, but having spent years on such projects, I can tell you to proceed slowly with caution. You need to do a lot of research to determine if you have systems, applications, or something else depending on other hosts, servers, printers, etc. having specific addresses, and such an inventory could take a long time and still miss things that could disrupt the business.

First you set up links between the two networks using a one-to-one NAT (Cisco has outside source NAT that is often used for that; see this answer about that). That should be a temporary situation until one or both of the networks get readdressed, but readdressing can take up to years with very large networks.

You need to create a plan of how to proceed, how to test, how to back out, etc.

You end up readdressing in chunks that are delimited by routers. If you want to readdress a network, you want to do an entire network behind a router at once. If the router has multiple networks attached, you typically do them one at a time, checking for problems after each change, and that can take some time. Trying to readdress too many networks at once can be a huge problem if mistakes are made, so you really want to limit what you do at each change, and give each change enough time (days or weeks) to make sure nothing is broken before doing another change.

The entire process can be very disruptive, especially if some applications or systems have hard-coded addresses pointing to hosts that have had their addressing changed.

Ron Maupin
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  • Generally, using DNS names throughout, possibly in combination with resilient DHCP can make this tedious process a breeze... – Zac67 Dec 21 '21 at 05:06
  • @Ron Maupin Wow thats a lot of information, I'll have to check out the links because I usually understand things a bit better visually. You know if I wanted to put the old and new pcs on different networks and route them manually, is there a tutorial or link on how to do this in a step by step and more simple manner since I am still new to this and learning, thanks! – JoeB Dec 21 '21 at 05:49
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    I don't know if this is a theoretical question or if you're actually involved in such a project, but can you give us a sense of the size of these companies? Are we talking about tens or hundreds of PCs? The network topology matters too. – Ron Trunk Dec 21 '21 at 13:40
  • @Zac67, The last time I did something like that, it involved over 100,000 users, and all the associated devices (servers printers, etc.). It actually required new WAN circuits at the sites that were converting. I think it took about two years, and it was a giant pain. We are now big enough that we are not allowed any more growth, so I should not need to deal with something like that again, but even a decade later, addressing stuff sometimes still pops up from that acquisition. – Ron Maupin Dec 21 '21 at 16:31