Learn in the textbookFundamentals of Access Control and Network Defense Devices
DefinitionA firewall (ファイアウォール) sits mainly at the boundary between an organization's network and the outside, or at a device's entrance, passing only traffic that matches set rules and blocking everything else.
ExplanationIt decides mainly by the source and destination addresses and the port numbers (numbers that identify the type of service) of each communication. Firewalls can divide a network to create an intermediate zone, the DMZ, that holds only the servers made public to the outside. Software installed on each PC to protect that machine is called a personal firewall (パーソナルファイアウォール).
AnalogyLike the auto-locking front door of an apartment block. Residents and the visitors they buzz in get through; everyone else stops at the door.
Memory hookIn a building, a fire wall stops a fire spreading from one section to the next, and the network version takes its name from it.