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

2 Firewall

A firewall protects networked computers from an intrusion that could compromise confidentiality or result in data
corruption or denial of service. It must have at least two network interfaces, one for the network it is intended to
protect, and one for the network it is exposed to. A firewall sits at the junction point or gateway between the two
networks, usually a private network and a public network such as the Internet.
A firewall examines all traffic routed between the two networks to see if it meets certain criteria. If it does, it is
routed between the networks, otherwise it is stopped. A firewall filters both inbound and outbound traffic. It can
also manage public access to private networked resources such as host applications. It can log all attempts to
enter the private network and trigger alarms when hostile or unauthorized entry is attempted. Firewalls can filter
packets based on their source and destination addresses and port numbers. This is known as address filtering.
Firewalls can also filter specific types of network traffic. This is known as protocol filtering because the decision
to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also
filter traffic by packet attribute or state.
It is important to note that an Internet firewall cannot prevent individual users with modems from dialling into or
out of the network. By doing so they bypass the firewall altogether and open the network to attack. However, these
are management issues that should be raised during the planning of any security policy and cannot be solved with
Internet firewalls alone.
NB714 or NB712
G.SHDSL Modem Router
(Note: NB714 model shown)
PC PC
8
Access to Specific Destination
Allowed Traffic
Restricted Traffic
PC
PC
Unknown Traffic
Specified Allowed Traffic
Out to Internet
Firewall
Internet
NB712 / NB714 User Guide
YML829 Rev1