Firewall..

what is a firewall??? maybe we do not know what it is. So, i want to share definition and how it works,.           
f+r´wâl) (n.) A system designed to prevent unauthorized 
access to or from a private network. Firewalls can b implemented in both hardware and software, or a 
combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing
private networks connected to the Internet, especially  
intranets. All messages entering or leaving the intranet
 pass through the firewall, which examines each message
 and blocks those that do not meet the specifiedsecurity criteria. There are several types of firewall techniques:

Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.

Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.

Circuit-level gateway: Applies security mechanisms when a TCP or UDPconnection is established. Once the connection has been made, packets can flow between the hosts without further checking.

Proxy server: Intercepts all messages entering and leaving the network. Theproxy server effectively hides the true network addresses.

In practice, many firewalls use two or more of these techniques in concert. A firewall is considered a first line of defense in protecting private information. For greater security, data can be encrypted.
f+r´wâl) (n.) A system designed to prevent unauthorized access to or from a privatenetwork. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internetusers from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified securitycriteria. 
There are several types of firewall techniques:

Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.

Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.

Circuit-level gateway: Applies security mechanisms when a TCP or UDPconnection is established. Once the connection has been made, packets can flow between the hosts without further checking.

Proxy server: Intercepts all messages entering and leaving the network. Theproxy server effectively hides the true network addresses.