They are either rejected or allowed to pass depending on if the packet’s information passes a set of filtering rules. Packet filtering firewalls worked on the first three layers of the OSI reference model, meaning that the majority of the work was done between the network and physical layers.
This technology has continually developed and evolved into today’s network layer firewalls – also called packet filters. One of a number of common firewall techniques, packet filter firewalls work at a low level of the TCP/IP protocol stack which use an established rule set as a means of blocking or allowing packets to pass into the network. Although effective, packet filtering can be susceptible to IP spoofing.
Another common firewall technique is an application gateway, which applies security mechanisms to specific applications such as FTP. Circuit-level gateway, another firewall technique, only applies security mechanisms when a TCP or UDP connection is established. Once a connection has been established, packets can flow without further monitoring or security checks.
Another popular firewall technique is the proxy server which intercepts all messages in and out of the network. Effectively, the proxy server hides all network addresses.
Standard Definition Exists?
A part of a computer system or network which is designed to block unauthorized access while permitting outward communication.