What are the basic types of firewalls?

Conceptually, there are two types of firewalls:

1.Network layer

2.Application layer

Network layer firewalls
These generally make their decisions based on the source, destination addresses and ports
in individual IP packets
These Firewall maintain internal information about the state of connections passing through them, the contents of some of the data streams, and so on.

Application layer firewalls
These generally are hosts running proxy servers, which permit no traffic directly between networks, and which perform elaborate logging and auditing of traffic passing through them.

Application layer firewalls can be used as network address translators, since traffic goes in one ``side'' and out the other, after having passed through an application