Question

Imagine a situation, I have PC with two lan cards, one is connected to internet another is connected to local network, how can I detect IP which is connected to internet with C# ?

Answer 1

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 http://www.example.com, so that it works even when the DNS server is down.