What is the C# code to determine a visitor's IP address?

To determine a person actual IP address first check the server variable HTTP_X_FORWARDED_FOR:

HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"]

If this returns nothing, check the IP address of the server or proxy of the person visiting by checking the server variable REMOTE_ADDR:

HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"]

As an alternative to using REMOTE_ADDR, you may also use "UserHostAddress". The "UserHostAddress" property acts as a wrapper for REMOTE_ADDR:

HttpContext.Current.Request.UserHostAddress

What is the code for a function to determine a visitor's IP address?

Here is some example C# code:

public string Ip()
{
    string ipAddress = Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
    if (ipAddress == null || ipAddress == "")
    {
        ipAddress = Request.ServerVariables["REMOTE_ADDR"];
    }
    return ipAddress;
}