Filter
Top Products
12-6 CHAPTER 12: CREATING AND USING PACKET FILTERS
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.
Figure 12-1 Ethernet and FDDI Packet Fields
Destination
Address
(6 octets)
Source Address
(6 octets)
Type/Length
(Ethernet Type field if > 1500;
802.3 Length field if ≤ 1500)
0
6
12 14 25
octets
Ethernet Packet
Data
(≤ 64 octets of data can be
examined by the filter)
0
6
12
25
octets
FDDI Packet
Destination
Address
(6 octets)
Source
Address
(6 octets)
Data
(≤ 64 octets of data can be
examined by the filter)