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What is a firewall?
A firewall is a system designed to reinforce the Security of the data flowing between two Networks, the Internal Network and the outside Network. There are several ways to accomplish this but most firewalls use two or more of the following methods as none of them alone provides adequate security.
Works at the Internet protocol layer and enables you to accept, reject or drop packets based on IP Address, Ports or Protocols. Packet filters perform these duties based on a set of configurable rules called Policies. Packet filtering is the original and the most basic type of firewalling and most routers provide packet filtering. Disadvantages of packet filtering however are:
Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.
Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.