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 level firewalls are application-specific proxies. Stateful multilayer firewalls combine aspects of the other three, filtering packets, checking sessions, and evaluating application layer contents. The document also discusses firewall environments like DMZs, intranets, extranets, and the future of increasingly sophisticated firewall technology.