GRT-504 4-Wire G.SHDSL.bis Firewall Router User’s Manual
3.2
Types of Firewall
There are three types of firewall:
3.2.1 Packet Filtering
In packet filtering, firewall will examine the protocol and the address information in the header of
each packet and ignore its contents and context (its relation to other packets and to the intended
application). The firewall pays no attention to applications on the host or local network and it
"knows" nothing about the sources of incoming data. Filtering includes the examining on incoming
and outgoing packets, and determines the packet dropping or not by a set of configurable rules.
Network Address Translation (NAT) routers offer the advantages of packet filtering firewalls but
can also hide the IP addresses of computers behind the firewall, and offer a level of circuit-based
filtering.
Level 5: Application
Level 4: TCP
Level 3: IP
Level 2: Data Link
Level 1: Physical
Protocol
Source/Destination address
Source/destination port
IP options
connection status
192.168.0.5
172.16.3.4
UDP
SP=3264
SA=192.168.0.5
DP=1525
DA=172.16.3.4
Filter remember this
information
UDP
SP=1525
SA=172.16.3.4
DP=3264
DA=192.168.0.5
Stateful Inspection
Matches outgoing so allows in
Nomatches so disallows in
UDP
SP=1525
SA=172.168.3.4
DP=2049
DA=192.168.0.5
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