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68 INTELLIGENT COMMUNICATION SYSTEMS

FIGURE 7.12 Intranet example.

An IP filter provides functions of packet control using this information. It checks
which packet is allowed to pass through the firewall. Usually a packet issued from
inside the intranet is allowed to pass through the net, but a packet from outside will
be checked carefully to see whether it should be allowed to pass or not. When IP
filtering rules are implemented, the router picks up a rule and compares the header
of a packet with it. When the rule and the header are coincident, the packet is denied
passage through the firewall. If they are not coincident, the router picks up the next
rule and compares the header with it. When they are coincident, the packet is
rejected. This procedure continues until the end of the rules.

7.7.2 Intranet Protocols
There are two intranet protocols: CSMA/CD and token ring. These are described
earlier in the chapter. This section describes the features.

7.7.2.1 CSMA/CD
This topology was invented by Xerox Corp. It is a local area network connected
by a bus interface named Carrier Sense Multiple Access/Collision Detection
(CSMA/CD). Here's how this method works: Any workstation on the intranet may
issue messages at any time, meaning that more than one message can be issued at