how to determine whether an IP address in private?

A more detailed response is here:

private bool _IsPrivate(string ipAddress)
{
    int[] ipParts = ipAddress.Split(new String[] { "." }, StringSplitOptions.RemoveEmptyEntries)
                             .Select(s => int.Parse(s)).ToArray();
    // in private ip range
    if (ipParts[0] == 10 ||
        (ipParts[0] == 192 && ipParts[1] == 168) ||
        (ipParts[0] == 172 && (ipParts[1] >= 16 && ipParts[1] <= 31))) {
        return true;
    }

    // IP Address is probably public.
    // This doesn't catch some VPN ranges like OpenVPN and Hamachi.
    return false;
}

The private address ranges are defined in RFC1918. They are:

• 10.0.0.0 - 10.255.255.255 (10/8 prefix)
• 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
• 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

You might also want to filter out link-local addresses (169.254/16) as defined in RFC3927.

The best way to do this would be an extension method to the IP Address class

    /// <summary>
    /// An extension method to determine if an IP address is internal, as specified in RFC1918
    /// </summary>
    /// <param name="toTest">The IP address that will be tested</param>
    /// <returns>Returns true if the IP is internal, false if it is external</returns>
    public static bool IsInternal(this IPAddress toTest)
    {
        if (IPAddress.IsLoopback(toTest)) return true;
        else if (toTest.ToString() == "::1") return false;

        byte[] bytes = toTest.GetAddressBytes();
        switch( bytes[ 0 ] )
        {
            case 10:
                return true;
            case 172:
                return bytes[ 1 ] < 32 && bytes[ 1 ] >= 16;
            case 192:
                return bytes[ 1 ] == 168;
            default:
                return false;
        }
    }

Then, one may call the method on an instance of the IP address class

    bool isIpInternal = ipAddressInformation.Address.IsInternal();