Request.ServerVariables["REMOTE_ADDR"].ToString()

Veya Aşağıdaki fonksiyonu kullanabilirsiniz.

public string ipcek()
    {
        string ipaddress="";
        try
        {
           ipaddress =  HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
        if (ipaddress == "" || ipaddress == null)
        {
            ipaddress = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];

        }
        }
        catch
        {
           
        }
        finally
        {

        }
        return ipaddress;
    }