The largest TCP/IP network port number allowable for IPv4

Go here and pick a port with the description Unassigned

Here is what I was looking for:

static int FreeTcpPort()
{
  TcpListener l = new TcpListener(IPAddress.Loopback, 0);
  l.Start();
  int port = ((IPEndPoint)l.LocalEndpoint).Port;
  l.Stop();
  return port;
}