gistfile1.cs

    public static bool IsInSubnet(this IPAddress address, string subnets)
    {
      if (String.IsNullOrEmpty(subnets))
        return false; // default allowed or not ?

      if (address.AddressFamily == System.Net.Sockets.AddressFamily.InterNetworkV6)
      {
        return false;
      }
      var subnet_splitted = subnets.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries);

      return subnet_splitted.Any(subnet =>
      {
        var parts = subnet.Split('/');
        var target = IPAddress.Parse(parts[0]);
        IPAddress mask = new IPAddress(mask32);
        if (parts.Length > 1)
          IPAddress.TryParse(parts[1], out mask);

        IPAddress network1 = address.GetNetworkAddress(mask);
        IPAddress network2 = target.GetNetworkAddress(mask);

        return network1.Equals(network2);
      });
    }