If site 'a' has same IP as that of site 'b' does that mean that they are hosted in the same server? What happens if site 'a' is a bad neighborhood site and site 'b' is a good site? Is there a good thread that may answer all these questions?
As with most things, it's not quite black and white as to whether site 'b' would be tainted by site 'a' purely because they are on the same class C or same IP address. Some IP addresses host tens of thousands of sites.