The firewall is part of a computer system or network that is designed to prevent unauthorized access while allowing authorized contacts. A device or devices configured to permit, deny, encrypt, disclose, or proxy to all (inside and outside) your computer traffic between different security domains, it is based on a set of rules and eligibility other criteria.
Firewalls can be implemented either in hardware or software, or a combination of both. Firewalls are often used to prevent unauthorized Internet users access private networks connected to the Internet, in particular, avoid intranets. All messages entering or leaving the intranet pass through the firewall, where each message and blocks those that do not meet specific safety standards.
Packet filtering: Packet filtering inspects each packet through the network, and accept or reject it under rules set by the user. Although it is difficult to form, is very effective, very clear to users. It is likely to mislead the copyright.
Implementation of the words: Applies security mechanisms to specific applications such as Telnet and FTP servers. This is very effective, but can impose performance degradation.
Circuit - the level of words applies security mechanisms when a TCP or [UDP connection. When the connection has been made, packets can flow between the hosts any further testing.
Proxy server: monitor all messages entering and leaving the network. The proxy server effectively hides the true network addresses.