ResolverController.cs

    public class ResolverController : ApiController
    {
        private const string ConnectionString = "DbConnectionString";
        private SqlConnection m_Connection;

        [CORSApiFilter]
        public JsonResult<Redirect> Get()
        {
            //Get the client IP
            var foundIP = "";
            var context = HttpContext.Current;
            var ipAddress = context.Request.Headers["X-Forwarded-For"];

            if (!string.IsNullOrEmpty(ipAddress))
            {
                var addresses = ipAddress.Split(',');

                if (addresses.Length != 0)
                    foundIP = addresses[0];
            }
            else
                foundIP = context.Request.ServerVariables["REMOTE_ADDR"];

            //Get the IP ranges from the DB
            var foundRedirects = this.GetRedirects();


            Redirect matched = null;
            var db = new DBHelper();

            foreach (var redirect in foundRedirects)
            {

                if (IsInRange(foundIP, redirect.StartIp, redirect.EndIp))
                {
                    matched = redirect;
                    break;
                }
            }
            return Json(matched);
        }

        /// <summary>
        /// Get redirects from the DB.
        /// </summary>
        private List<Redirect> GetRedirects()
        {
            var connectionString = ConfigurationManager.ConnectionStrings[ConnectionString].ConnectionString;
            m_Connection = new SqlConnection(connectionString);

            var loadedRedirects = new List<Redirect>();

            using (var cmd = new SqlCommand("SELECT RegistrationId, StartIp, EndIp, RedirectName FROM dbo.Registration", this.m_Connection))
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection.Open();
                var rdr = cmd.ExecuteReader();

                while (rdr.Read())
                {
                    var newRedirect = new Redirect
                    {
                        RegistrationId = Guid.Parse(rdr[0].ToString()),
                        StartIp = rdr[1].ToString(),
                        EndIp = rdr[2].ToString(),
                        RedirectName = rdr[3].ToString()
                    };

                    loadedRedirects.Add(newRedirect);
                }
                rdr.Close();
                cmd.Connection.Close();
            }
            loadedRedirects.Sort((x, y) => x.StartIp.CompareTo(y.StartIp));
            return loadedRedirects;
        }

        /// <summary>
        /// Function returning true if in IP is within the specified ip range.
        /// </summary>
        /// <param name="ipAddress">The ip to check.</param>
        /// <param name="fromAddress">Start IP of the range.</param>
        /// <param name="toAddress">End IP of the range.</param>
        /// <returns>true if ip is in range otherwise false.</returns>
        private static bool IsInRange(string ipAddress, string fromAddress, string toAddress)
        {
            var ip = IpAddressToLongBackwards(ipAddress);
            return ip >= IpAddressToLongBackwards(fromAddress) && ip <= IpAddressToLongBackwards(toAddress);
        }

        // Convert IPAddress to long backwards for comparison.
        private static long IpAddressToLongBackwards(string ipAddress)
        {
            IPAddress ip;
            if (!IPAddress.TryParse(ipAddress, out ip))
                throw new ArgumentException(string.Format("The value '{0}' could not be parsed as an IP address.",
                    ipAddress));

            var byteIp = ip.GetAddressBytes();
            var longIp = (long)byteIp[0] << 24;
            longIp += (long)byteIp[1] << 16;
            longIp += (long)byteIp[2] << 8;
            longIp += byteIp[3];

            return longIp;
        }
    }