An IP address is the location of a computer/server on the Internet. It is 4 sets of digits, known as octets.
The third octet is called the C-Class IP address
For example 211.125.125.91
125 is the C-Class IP address.
Generally webhosts assign a certain number of IP addresses for each server, and when you buy your shared hosting account you are getting a shared IP address. If you get a reseller hosting account they will assign you maybe a couple of IP addresses. Therefore, you have to ensure that your assigned IP addresses have different C-classes. The easiest way to do that is to use different web hosts, or if you have a dedicated server you can get your web hosting provider to assign you IP addresses from different C-classes (if they are a data center like The Planet – and not just someone selling boxes in a data center that should not be an issue).
Where you get into a real issue, is where you are hosting inter-linked websites on basically a cheap shared hosting plan such as what we talked about earlier in this thread. You probably are on the same IP-address for all your domains or worse yet…you are basically using sub-domains in your structure that are being assigned/redirected to primary top level domain addresses. This is another topic altogether.
Another issue that EWhitaker indicated and it is an excellent point to make… if you are going to use shared/virtual hosting make sure you at least have your own IP address in case one of the other users on that server is doing something like spamming or wasting the resources of that server. The web host will cut-off the IP address. However, you are still at risk with shared/virtual hosting of your server going down by others on the same box using all the resources. This is the risk of shared hosting.
One of the factors we have been talking about here is that search engines determine your ‘importance’ according to the number of other websites and webpages linking to you. There are other factors that determine your ranking in the search engines but this is one of the major factors. Because of this fact, webmasters often set up multiple websites with similar content and link them together to achieve higher-rankings in the search engines.
Such sites have been known to be called sister sites, parallel sites, etc.
If you are using the same hosting provider for multiple websites i.e. like reseller hosting or “hostmonster”, etc, the probability is that your sites will be on the same IP address or the same C-class IP block.
If you are going to run sister/parallel sites, make sure they have different content and try to ensure those sites are on different C-Class IP addresses.
Here is some more detail in case you are interested
xxhttp://www.searchguild.com/redir/o.php?out=http://computer.howstuffworks.com/question549.htm
Good Article
xxhttp://www.theinternetdigest.net/articles/ip-address-overview.html