User manual

8-20 Packet Filter

Filter Examples

This section provides examples that will help you build

commonly used filters.

This section describes the following topics:

• IP Packet Filter Rule Examples

• IPX Packet Filter Rule Examples

• AppleTalk Packet Filter Rule Examples

IP Packet Filter Rule Examples

This section briefly describes IP packet filtering options, and

provides rule examples for each IP packet filtering capability.

This section includes the following topics:

• Source and Destination Address Filtering

• Masks

• TCP and UDP Parameter Filtering

• IP RIP Packet Filtering

• ICMP Packet Filtering

• IP Call Filtering

Source and Destination Address Filtering

Source and destination address filtering is generally used to

limit permitted access to trusted hosts and networks only, to

explicitly deny access to hosts and networks that are not trusted,

or to limit external access to a given host (for example, a Web

server or a firewall).

Note that only the part of the IP address specified by the mask

field is used in the comparison. If a match is found, the packet

is forwarded (rules containing accept) or discarded (rules

containing reject).

The following rule example allows forwarding of IP packets

with source addresses that match the first 16 bits of the given IP

address (that is, addresses beginning with 192.77):