If you've never had to deal with networks with overlapping or duplicated IP ranges this you may also be the kind of person that's prone to winning at slot machines and always seems to be able to catch a cab in the rain - i.e. you're one lucky propeller head. Managing networks where the same address ranges appear in multiple places is a read pain. In enterprise networks this usually occurs when you go through M&A.
I used to run the technical side of M&A for a large company here in the US that, at the time, gobbled up companies the way that many of us will be gobbling up turkey, dressing, and pumpkin pie next week and there are some great best practices for each. With regards to the Thanksgiving feast, keep these things in mind:
1. Wear something that stretches. Elastic is good - zippers are bad. 2. Pace yourself. This is especially important if your Thanksgiving involves multiple meals at multiple locations. 3. Skip the stuff that you don't especially like so that you can eat more of the stuff you love. 4. Remember that you can take a break between dinner and desert. I like to call it half time. 5. Have no remorse. Not on this day. No way no how.
Now, on the technical side of M&A there are also some great best practices. Probably the most important from a network engineering standpoint is that you need to plan on re-addressing the networks of the companies you're buying if they don't fit in with your existing addressing schemes and practices. Yes, this is a pain but it is a much smaller pain than trying to manage networks with overlapping address ranges.
In service provider networks, especially MSPs, this isn't so simple. You can't demand that your customers all re-address their networks just so that they'll be easier for you to manage. If your customers are small businesses, it's even worse. There's a pretty good chance that they're all using some parts of the 192.168.x.x network and you can figure they all started with 192.168.0.0/24 and 192.168.1.0/24.
If you find yourself in the situation of managing networks with duplicated IP address ranges and re-addressing isn't an option, there are some things that you can do to minimize the pain. First, be sure that you choose an NMS that has a solution for this type of thing. If you ask your network management vendor about overlapping IP address space and you get a blank stare that's a bad sign. Second, really think through your NAT. The more you can do one-to-one the easier your job will be (both now and if you ever decide to re-address). And third, be sure that your boss understands why this is a bad thing so that your hard work is appreciated and he'll be prepared when you need to spend some cash to work within this situation.
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