Windows Service

I have a simple Client / Server example written in VS 2005 C#.  I am testing both on the _same_ XP SP2 machine.  I am trying to make the Client send the Server a UDP message.  In the CLient, if I use my loopback address like below, all works as expected.  If I instead use my actualy IP address in the Client, then it does not work.  But I need the UDP packet to actually go across the wire.  Any ideas?

Works : IPAddress broadcast = IPAddress.Parse("127.0.0.1");
Does not Work:  IPAddress broadcast = IPAddress.Parse(_myIPAdrress_);