IsInternal IPAddress Extension

IPAddressExtensions.cs

using System;
using System.Net;
using System.Net.Sockets;

static class IPAddressExtensions
{
    public static bool IsInternal(this IPAddress address)
    {
        if (address == null)
            throw new ArgumentNullException(nameof(address));

        if (address.AddressFamily != AddressFamily.InterNetwork && address.AddressFamily != AddressFamily.InterNetworkV6)
            throw new ArgumentException("Only IPv4 & IPv6 addresses are supported", nameof(address));

        if (IPAddress.IsLoopback(address))
            return true;

        if (address.AddressFamily == AddressFamily.InterNetwork)
        {
            // Private IPv4 Addresses
            // 10.0.0.0 - 10.255.255.255
            // 172.16.0.0 - 172.31.255.255
            // 192.168.0.0 - 192.168.255.255

            byte[] addressBytes = address.GetAddressBytes();

            switch (addressBytes[0])
            {
                case 10:
                    return true;
                case 172:
                    return addressBytes[1] >= 16 && addressBytes[1] < 32;
                case 192:
                    return addressBytes[1] == 168;
                default:
                    return false;
            }
        }
        else if (address.AddressFamily == AddressFamily.InterNetworkV6)
        {
            // Unique Local IPv6 Unicast Addresses - See https://www.rfc-editor.org/rfc/rfc4193.txt
            // fc00::/7
            byte[] addressBytes = address.GetAddressBytes();
            return addressBytes[0] == 0xFC || addressBytes[0] == 0xFD;
        }

        return false;
    }
}

TestIsInternal.cs

using System;
using System.Net;

class Program
{
    static void Main()
    {
        PrintIsInternal("127.0.0.1"); //True
        PrintIsInternal("10.0.0.1"); //True
        PrintIsInternal("172.16.0.1"); //True
        PrintIsInternal("192.168.0.1"); //True
        PrintIsInternal("fd04:5824:642e:2c13::"); //True

        //result of nslookup for www.google.com: 2a00:1450:4006:805::2004
        PrintIsInternal("2a00:1450:4006:805::2004"); //False
        PrintIsInternal("1.2.3.4"); //False
    }

    static void PrintIsInternal(string ip)
    {
        Console.WriteLine($"IsInternal for {ip} returns {IPAddress.Parse(ip).IsInternal()}");
    }
}