The document discusses different types of firewalls and their environments. It describes packet filter, circuit level, application level, and stateful multilayer firewalls. Packet filters work at the network layer and compare packets to criteria. Circuit level firewalls monitor TCP handshaking. Application proxies are application specific. Stateful multilayer firewalls combine aspects of the other three, filtering packets, checking sessions, and evaluating application layer contents. Firewalls are important for network security but cannot provide all security functions alone.