A firewall is a piece of hardware or software that prevents data packets from either entering or leaving a specified network. To control the flow of traffic, numbered ports in the firewall are either opened or closed to specific types of packets. The firewall reviews two pieces of information in each arriving or departing packet: the protocol through which the packet is being delivered, and the port number to which it is being sent. If the firewall is configured to accept the specified protocol through the targeted port, the packet is allowed through.
You can configure each control protocol plug-in (MMS, RTSP, and HTTP) to use a specific port to make firewall configuration easier. Therefore, if your network administrator has already opened a series of ports for use by your Windows Media servers, you can allocate those ports to the control protocols accordingly. If not, you can request that the default ports for each protocol be opened. If opening ports on your firewall is not possible, Windows Media Services can stream content by using the HTTP protocol over port 80.