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copied!<p>Try this:</p> <pre><code>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; } </code></pre> <p>Note that this implementation of <code>findMatch()</code> relies on class C matching. If you want to support class B matching, just omit the check for <code>ipBytes[2] == gatewayBytes[2]</code>.</p> <p><strong>Edit History:</strong></p> <ul> <li>Updated to use <code>www.example.com</code>.</li> <li>Updated to include <code>getInternetIPAddress()</code>, to show how to use the other methods.</li> <li>Updated to catch <code>FormatException</code> if <code>getInternetGateway()</code> failed to parse the gateway IP. (This can happen if the gateway router is configured such that it doesn't respond to <a href="http://en.wikipedia.org/wiki/Traceroute" rel="noreferrer">traceroute</a> requests.)</li> <li>Cited Brian Rasmussen's comment.</li> <li>Updated to use the IP for www.example.com, so that it works even when the DNS server is down.</li> </ul>

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