If one actually believes that IPs do the trick, the only way to do it properly would be to truly imitate real life. If you want your sites to appear to be hosted in different places, you need to use different hosts with servers in different locations. You'll also need to make them appear to be owned by different people/companies. Anything else, and the search engine, like a human, will be able to notice that some of the coincidences are strange.
Regardless, if you'll be interlinking 1000 sites, this can be determined algorithmically.
If you have 10 sites in related niches, and it makes sense to send visitors from one to the other, then there's little chance that you're risking being penalized by the search engines. It's the same with a company linking to its customer support forum. If it makes sense to do it for the visitors, do it.