Hi!

Use the GetAdaptersInfo Win32 API, via PInvoke, and get the Gateway IP
address. The following links should help you:

http://msdn.microsoft.com/library/en-us/iphlp/iphlp/getadaptersinfo.asp
http://support.microsoft.com/?kbid=223025
http://www.codeguru.com/network/ViewAdapterInfo.html

Take a look at the System.Management classes and the WMI class
Win32_NetworkAdapterConfiguration.

Here's sample to get you started.

using System;
using System.Management;
using System.Runtime.InteropServices;
using System.Threading;
public class Wmis {
public static void Main() {
//Index (here 9 as a sample) depends on the number of network adapters and
protocols bound to it.
// Use the Win32_NetworkAdapter class to get the index of the adapter for
which you need to query the configuration.

 using (ManagementObject netw = new
ManagementObject("Win32_NetworkAdapterConfiguration.Index=9"))
 {
  foreach(string gw in netw["DefaultIpGateway"] as string[]) // property is
a string array
      Console.WriteLine( "GateWay\t?{0}?", gw);
 }
}
}

Willy.