You can't get the IP address at the client side - you have to refer to the server. The Client side IP is a local address (192.xxx.xxx.xxx or similar) which does not extend past the router. Equally, the Server cannot see the Client actual IP - it sees the IP address of the actual router, which is assigned by the ISP.
The easiest way to do it is to ask the server:
<br />Your IP address is <%= Request.ServerVariables["REMOTE_ADDR"] %><br />