A firewall operating as a packet filter passes or blocks traffic to specific addresses based on the
type of application. The packet filter doesn’t analyze the contents of a packet; it decides whether
to pass it based on the packet’s addressing information. For instance, a packet filter may allow
web traffic on port 80 and block Telnet traffic on port 23. This type of filtering is included in
many routers. If a received packet request asks for a port that isn’t authorized, the filter may
reject the request or simply ignore it. Many packet filters can also specify which IP addresses can
request which ports and allow or deny them based on the security settings of the firewall