Types Of Firewall - NetComm G.SHDSL 4-port Security Modem Routers NB712 User Manual

2.1 Types of Firewall

There are three types of firewall:
2.1.1 Packet Filtering
In packet filtering, only the protocol and the address information of each packet is examined. Its
contents and context (its relation to other packets and to the intended application) are ignored.
The firewall pays no attention to applications on the host or local network and it "knows" nothing
about the source of the incoming data. Filtering consists of examining incoming or outgoing packets
and allowing or disallowing their transmission on the basis of 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.
192.168.0.5
192.100.0.10:1025
192.100.0.11:4433
Internal/Protected
Network
NB712 / NB714 User Guide
YML829 Rev1
Level 5: Application
Protocol
Level 4: TCP
Source/Destination address
Source/Destination port
Level 3: IP
IP options
Connection status
Level 2: Data Link
Level 1: Physical
172.16.3.4
Firewall
Filter remembers
this information
UDP
SP=3264
SA=192.168.0.5
DP=1525
DA=172.16.3.4
Matches outgoing
so allowed
UDP
SP=1525
SA=172.16.3.4
DP=3264
DA=192.168.0.5
No matches
so disallowed
UDP
SP=1525
SA=172.168.3.4
DP=2049
DA=192.168.0.5
192.120.8.5:2205
192.120.8.5:2206
Firewall 192.120.8.5
Client IP Internal Port
192.68.0.10 1025
192.168.0.11 4406
Internet
External Port
2205
2206
External/Unprotected
Network
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