We have a couple of hundred websites and it was advised that 'preferably' (not a requirement, but preferably), the websites should be hosted on their own IP addresses (not shared IPs). Most (not all, i'd say about 20%) would be linking to each other (as they belong to the same vertical). My question is, when applying SEO techniques, would the search engines look at this and say Aha! adjacent IPs - must be a link farm - ban them? i.e. (example) domain1.com IP 192.168.1.1 domain2.com IP 192.168.1.4 domain3.com IP 192.168.1.7 domain4.com IP 192.168.1.10 domain5.com IP 192.168.1.18 domain6.com IP 192.168.1.21 domain7.com IP 192.168.1.25 Would such a scenario warrant an exclusion from the Big G? I must emphasize that its NOT a link farm, simply trying to cross-reference the websites that all belong to the same vertical.
You can host them all under the same IP on a dedicated server and have no issues ranking them properly or as if they all had there own unique IP address. Your just wasting money on the extra IP addresses. This is "why" people use unique address, because they use link schemes that violate the Google Webmaster Guidelines to manipulate search results by inter-linking their sites. You cannot inter-link your websites unless they contain the nofollow attribute. Yes, Google isn't stupid and can detect this "old" link farm technique. Then nofollow them all properly as the guidelines would state.
Yes the big G knows if you are on the same class C IP range and will pass little juice. Interlinking (reciprocal) will pass very little juice if not throw up a red flag.