How to get *internet* IP?

Try this:

static IPAddress getInternetIPAddress()
{
    try
    {
        IPAddress[] addresses = Dns.GetHostAddresses(Dns.GetHostName());
        IPAddress gateway = IPAddress.Parse(getInternetGateway());
        return findMatch(addresses, gateway);
    }
    catch (FormatException e) { return null; }
}

static string getInternetGateway()
{
    using (Process tracert = new Process())
    {
        ProcessStartInfo startInfo = tracert.StartInfo;
        startInfo.FileName = "tracert.exe";
        startInfo.Arguments = "-h 1 208.77.188.166"; // www.example.com
        startInfo.UseShellExecute = false;
        startInfo.RedirectStandardOutput = true;
        tracert.Start();

        using (StreamReader reader = tracert.StandardOutput)
        {
            string line = "";
            for (int i = 0; i < 9; ++i)
                line = reader.ReadLine();
            line = line.Trim();
            return line.Substring(line.LastIndexOf(' ') + 1);
        }
    }
}

static IPAddress findMatch(IPAddress[] addresses, IPAddress gateway)
{
    byte[] gatewayBytes = gateway.GetAddressBytes();
    foreach (IPAddress ip in addresses)
    {
        byte[] ipBytes = ip.GetAddressBytes();
        if (ipBytes[0] == gatewayBytes[0]
            && ipBytes[1] == gatewayBytes[1]
            && ipBytes[2] == gatewayBytes[2])
        {
            return ip;
        }
    }
    return null;
}

Note that this implementation of findMatch() relies on class C matching. If you want to support class B matching, just omit the check for ipBytes[2] == gatewayBytes[2].

Edit History:

• Updated to use www.example.com.
• Updated to include getInternetIPAddress(), to show how to use the other methods.
• Updated to catch FormatException if getInternetGateway() failed to parse the gateway IP. (This can happen if the gateway router is configured such that it doesn't respond to traceroute requests.)
• Cited Brian Rasmussen's comment.
• Updated to use the IP for www.example.com, so that it works even when the DNS server is down.

I suggest this simple code since tracert is not always effective and whatsmyip.com is not specially designed for that purpose :

private void GetIP()
{
    WebClient wc = new WebClient();
    string strIP = wc.DownloadString("http://checkip.dyndns.org");
    strIP = (new Regex(@"\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b")).Match(strIP).Value;
    wc.Dispose();
    return strIP;
}

The internet connection must be on the same IP network as the default gateway.

There's really foolproof no way to tell from the IP address if you can reach the "internet" or not. Basically you can communicate with your own IP network. Everything else has to go through a gateway. So if you can't see the gateway, you're confined to the local IP network.

The gateway, however, depends on other gateways, so even if you can access the gateway, you may not be able to reach some other network. This can be due to e.g. filtering or lack of routes to the desired networks.

Actually, it makes little sense to talk about the internet in this sense, as you will probably never be able to reach the entire internet at any given moment. Therefore, find out what you need to be able to reach and verify connectivity for that network.

This is my attempt to get the default IPv4 address without having to resort to DNS or external process calls to commands like ipconfig and route. Hopefully the next version of .Net will provide access to the Windows routing table.

public static IPAddress GetDefaultIPv4Address()
{
    var adapters = from adapter in NetworkInterface.GetAllNetworkInterfaces()
                   where adapter.OperationalStatus == OperationalStatus.Up &&
                    adapter.Supports(NetworkInterfaceComponent.IPv4)
                    && adapter.GetIPProperties().GatewayAddresses.Count > 0 &&
                    adapter.GetIPProperties().GatewayAddresses[0].Address.ToString() != "0.0.0.0"
                   select adapter;

     if (adapters.Count() > 1)
     {
          throw new ApplicationException("The default IPv4 address could not be determined as there are two interfaces with gateways.");
     }
     else
     {
         UnicastIPAddressInformationCollection localIPs = adapters.First().GetIPProperties().UnicastAddresses;
         foreach (UnicastIPAddressInformation localIP in localIPs)
         {
            if (localIP.Address.AddressFamily == AddressFamily.InterNetwork &&
                !localIP.Address.ToString().StartsWith(LINK_LOCAL_BLOCK_PREFIX) &&
                !IPAddress.IsLoopback(localIP.Address))
            {
                return localIP.Address;
            }
        }
    }

    return null;
}

A hacky way is to fetch and scrape one of the many 'What Is My IP' type websites.