A “Firewall” is a network component that connects an internal network (Intranet) to a public network (e.g. Internet), in much the same way as a Router. Access to the other network can be limited or blocked completely. This limitation/blocking will depend on the direction of access, the used utility and authentication/identification of the network subscriber.
Another feature of a Firewall can be the encryption of data. For example, this may make sense if the public network is being used only as a transit route between two physically remote parts of an Intranet.
