PCs send their requests to the Internet via the firewall, which acts as a representative of the PCs to the Internet. The target system therefore sees only the firewall, which in turn forwards the response packets of the target system to the appropriate PC on the internal network.
Depending on the type of firewall it is able to analyze the network packets in both directions and filter them, even before they reach the actual communication partner. Connection of an external firewall that restricts access between the Internet and the private (self-contained) network as handled by the Computer Service Centre Sydney.
Example configuration: The internet can be accessed via DSL connection, including DSL modem. The firewall could then be installed on a DSL router, which is addressed by the PCs in the private (self-contained) network as a default gateway. The device managed by the network requests the internal PCs and can distinguish between requests to the internal (private) and external network (Internet) and forward them accordingly.