There are three traditional types of firewalls:
packet filters
Packet filters are hardware firewalls (implemented in bridges or routers) that examine the source or destination address of a data packet to determine whether it should be forwarded to the next segment of a network.
application servers
Application servers are software firewalls that exist only to support particular applications (such as e-mail or a Web server) and grant access to such services according to guidelines set by the system manager who configures the application.
circuit-level gateways
Software-based circuit-level gateways fall somewhere between the other two types of firewalls. They don't actually host network services like application servers do; rather, they grant access to such services based on the unique identity of the network port through which the request for service is received-like a packet filter routing data based on addresses.
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