The term "firewall" originally referred to fireproof walls that were designed to prevent the spread of fire from one room or building to the next. They were typically made of brick, steel, or another type of inflammable material that would effectively limit the spread of the fire. In the IT world, firewalls do not involve any fire or pyrotechnics, but they serve a similar purpose. A computer firewall limits the data that can pass through it and protects a networked server or client machine from damage by unauthorized users.
Firewalls can be either hardware or software-based. A router is a good example of a hardware device that has a built-in firewall. Most routers can be configured to limit traffic from certain IP addresses or block requests based on other criteria. Software programs that monitor and restrict external access to a computer or network can also serve as firewalls. A network firewall only allows authorized traffic from the Internet to flow in and out of the network.