Firewall

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.
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.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.