An implicit, or transparent proxy is invisible to the client. That is, the client does not explicitly identify a proxy server, but the application nevertheless sends client requests and server responses through a proxy. Implicit proxy is generally represented by a diagram in which a dotted line indicates that the proxy server is transparent to the client.
Windows Media Services does not implement implicit proxy functionality; however, implicit proxying can be accomplished by using Layer 4 or Layer 7 network switches.
Diagram illustrating ISA firewall as a transparent service.