Don't use Request.substring. It will not give the expected result which you require. Instead use this.
string [] test = link.Split('/');
string required_subpart = test[3].ToString();
Note: the index is 3 here. Why because the url contains http://. So if you do not have http:// in your url, the index would be 1. Substitute it with whatever part you want in a url.
Let me know if this works for you, or I have an alternate solution for you.