A firewall is a system enforces
an access control policy between two networks while sitting in-between the two
networks. The policy is to block one part of the traffic while permitting the
other part, based on the firewalls configuration. The firewall is implemented
in software and hardware levels.
Hardware Firewall
The hardware firewalls are
intended to protect every machine on a local network. It is available as a
stand-alone product and also found in broadband routers. A hardware firewall
can be effective with little or no extra configuration. The technique used by a
hardware firewall to defect threats is called packet filtering, which examines
the header of a packet to determine its source and destination addresses and
port. This information is compared to a set of rules that determine whether the
packet is to be forwarded or dropped. However, hardware firewall is
non-effective for email worms of Trojan horses.