yhan (1) [Avatar] Offline
The firewall definition is not that quite right in P.29 2.2.2. Enterprise layers add security and overhead. "A firewall is simply a machine placed between the Internet and the HTTP servers of a corporation..."

1) A firewall can be hardware and/or software.
2) It functions more than the Internet and HTTP servers.

Getting a definition from a computer dictionary may be a good choice.