Question

How can you tell the difference between a request going to 127.0.0.1 and localhost.

This line of code on Windows 7 and VS2010 built-in web server can not tell the difference.

if (Request.ServerVariables["SERVER_NAME"].ToLower() == "localhost")
{

}

try hitting your own built-in web server with:
http://127.0.0.1/
and then
http://localhost/

Answer 1

Request.Headers will differentiate the requests:

if (Request.Headers["host"].ToLower() == "localhost") 
{ 
  //shouldn't be hit for 127.0.0.1
} 

Note: depending on your needs, you will have to consider clearing off the port number before your check.