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chance to forward or reject it, and so on. Because of this hierarchical structure, each filter
is said to have a priority. The first filter has the highest priority, and the last filter has the
lowest priority.
How individual filters work
As described above, a filter applies criteria to an IP packet and then takes one of three
actions:
• Forwards the packet to the local or remote network
• Blocks (discards) the packet
• Ignores the packet
A filter forwards or blocks a packet only if it finds a match after applying its criteria. When
no match occurs, the filter ignores the packet.
A filtering rule
The criteria are based on information contained in the packets. A filter is simply a rule that
prescribes certain actions based on certain conditions. For example, the following rule
qualifies as a filter:
“Block all Telnet attempts that originate from the remote host 199.211.211.17.”
This rule applies to Telnet packets that come from a host with the IP address
199.211.211.17. If a match occurs, the packet is blocked.