Get the client IP address or forwarder IP address in C# ASP.NET.

        /// <summary>
        /// Attempts to retrieve the real client ip (w/ proxy detection).
        /// </summary>
        /// <returns>The IP address of the remote client,
        /// or "0.0.0.0" if it cannot be obtained.</returns>
        public static string GetClientIpAddress()
        {
            const string defaultValue = "0.0.0.0";

            HttpContext context;
            if ((context = HttpContext.Current) == null)
            {
                return defaultValue;
            }

            HttpRequest request;
            try
            {
                // store a local ref to the request object:
                request = context.Request;
            }
            catch (HttpException)
            {
                Debug.WriteLine("Http request context is not available.");
                return defaultValue;
            }

            //
            // check proxy forwarder indicators first:
            string forwarderForIp = request.ServerVariables["HTTP_X_FORWARDED_FOR"];
            if (!string.IsNullOrEmpty(forwarderForIp))
            {
                int firstCommaIndex = forwarderForIp.IndexOf(',');
                if (firstCommaIndex > 0)
                {
                    // X-Forwarded-For can return multiple ip's, so 
                    // just get the first one.
                    // ex: X-Forwarded-For: client, proxy1, proxy2
                    forwarderForIp = forwarderForIp.Substring(0, firstCommaIndex).Trim();
                }

                if (!forwarderForIp.Equals("unknown",
                    StringComparison.InvariantCultureIgnoreCase))
                {
                    return forwarderForIp;
                }
            }

            // get the client ip from server from server vars:
            string remoteAdd = request.ServerVariables["REMOTE_ADDR"];

            return !string.IsNullOrEmpty(remoteAdd) ? remoteAdd : defaultValue;
        }