Basic Elements Of A Packet Filter - 3Com SUPERSTACK 2200 User Manual

12-6 C 12: C U P F
HAPTER REATING AND SING ACKET ILTERS

Basic Elements of a Packet Filter

Before creating a packet filter, you must decide which part of the packet
you want to filter. You can filter Ethernet packets by the destination address,
source address, type/length, or some part of the data. You can filter FDDI
packets by the destination address, source address, or some part of the
data. A packet filter operates on these fields to make filtering decisions.
Ethernet and FDDI packet fields are shown in Figure 12-1.
Destination Type/Length
Address (Ethernet Type field if > 1500;
(6 octets) 802.3 Length field if 1500)
Ethernet Packet
octets
0 6 12 14 25
Data
Source Address
(6 octets) ( 64 octets of data can be
examined by the filter)
Destination Source
Address Address
(6 octets) (6 octets)
FDDI Packet
octets
0 6 12 25
Data
( 64 octets of data can be
examined by the filter)
Figure 12-1 Ethernet and FDDI Packet Fields