This might help you on what to use.
Here's the fiddle although it seems to experience intermittent connection issues at this time:
http://www.sqlfiddle.com/#!3/55cda0/15
And basically you can select values by using this code:
SELECT LEFT(IP_Address, LEN(IP_Address) - CHARINDEX('.',REVERSE (IP_Address))) FROM IP
First, it turns around the string, and it will look for the first (which is actually the last, ha!) dot (or whatever separator you set).
After this you just simply select the leftmost of the remaining string where it substracts the location of the separator from the total length; therefore getting rid of anything after the last separator character - which is in your case a dot.