A security mechanism that stands as a barrier between systems and/or networks. Firewalls are usually used between the Internet and a computer or a smaller network. The firewall provides protection by scrutinizing messages that are requesting access to the computer or the network and permitting entrance based on pre-established rules. Firewalls permit access according to one of two philosophies: they may either allow all messages except those that have been explicitly denied entrance, or they may deny access to all messages except those that have been explicitly allowed entrance.
There are several types of firewall techniques; most firewalls incorporate two or more of the following methods:
Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules.
Application-level : Judges messages based on the application they are requesting connection to, such as FTP and Telnet servers.
Stateful Packet Inspection: Inspects a series of packets to establish that a previous connection with the external computer has been made. If so, it will allow the packet entry. If not, the firewall will accept or deny access to the packet based on other rules.
Proxy Server: Intercepts all messages entering and leaving the network. The proxy server can effectively hide the true network addresses.
FTP or File Transfer Protocol
A set of rules for exchanging files between computers via the Internet.