How to trim IP Address to get the first 3 parts of it?

String result = input.Substring(0, input.LastIndexOf("."));

Using String.LastIndexOf(), it should be easy.

EDIT
Using that method you can locate the last '.'. Then you need a substring up to and (apparently) incuding that '.'. Something like:

string shortened = longIP.Substring(0,longIP.LastIndexOf(".")+1);

string ip= "192.168.1.100";
string partial = ip.Substring(0,ip.LastIndexOf("."));