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a) the shared server is misconfigured and it screws up the virtual hosting in some way
b) the is excessive web-spam websites from the same shared IP or IP Range - or if there is excessive cross-linking within the same IP/IP Range.
Point a is a given - and it will screw up many things - not just search robot visits. Point b is just commonsense and not a rule!
If I had just started out, I would probably go for the cheapest host I could find with a good reputation.
However, if I ran a 10 year old site, generating millions of dollars of sales, I would have a fully redundant, dedicated server.
Maybe, after you reach rank X, then you could upgrade your web hosting?
however, leo11877, i don't think "IP" makes difference. "server" does.
1) often 'they' can guess two IPs are on the same server, if they want (well, i think they can if IPs are in the same network).
2) if server is misconfigured, likely it will screw up virtual hosts as well.
3) a slow server is still slow, if you use a different IP on the same server. and you don't want a slow server.
4) sometimes you need to do some uncommon configuration on your server, and you have to have root access. in this case, you need your server.