How to get external IP in forms app inC#

Your router configuration uses Network Address Translation(NAT). This is handled within the router that is connected to the Internet provider. The PC does not know the external (Internet) address. Solution 1 and Solution 2 both rely on an external server to echo the client address that it sees. Using either of these two techniques will mean that your software is dependent on an external web server run by volunteers.

The code below does not answer your question but this is how I get the IP address of the PC running my application. It is the IP address of the PC on the local area network. It is not the IP address of the router.
string strHostName = System.Net.Dns.GetHostName;
string strIPAddress = null;
System.Net.IPAddress[] objAddressList = System.Net.Dns.GetHostEntry(strHostName).AddressList;
for (x = 0; x <= objAddressList.GetUpperBound(0); x++) {
if (objAddressList(x).AddressFamily == Net.Sockets.AddressFamily.InterNetwork) {
strIPAddress = objAddressList(x).ToString;
}
}