The firewall is probably the best known security appliance. By definition firewall is a system or a group of systems which implements access policy between two or more networks.
Firewalls can be classified into four main classes:
1. Dedicated firewalls
2. Routers integrated firewalls
3. Servers integrated firewalls
4. Personal firewalls
1. Dedicated firewalls are hosts that runs an operating system designed for packet filtering and addresses translation. We can exemplify PIX systems or Checkpoint. These systems are capable of sustaining a large number of connections but routing facilities are extremely limited. For a simple network , firewall can be used as a router. For more complex networks is necessary a router.
2. Firewalls integrated into routers are used to remove the previous insufficiency. This class can not sustain the same number of connections, but it does better in more complex topologies, where you need the facilities of a router. Many products provide routers integrated firewall facilities, from firewall modules for high-end routers, to extremely compact dedicated for use in SOHO networks.