Different types of firewalls

1.Packet filtering firewalls

This, the original type of firewall, operates inline at junction points where devices such as routers and switches do their work.However, this firewall doesn't route packets, but instead compares each packet received to a set of established criteria -- such as the allowed IP addresses, packet type, port number, etc.

2.Circuit-level gateways

Using another relatively quick way to identify malicious content, these devices monitor the TCP handshakes across the network as they are established between the local and remote hosts to determine whether the session being initiated is legitimate -- whether the remote system is considered trusted. They don't inspect the packets themselves, however.

3.Stateful inspection firewalls

State-aware devices, on the other hand, not only examine each packet, but also keep track of whether or not that packet is part of an established TCP session. This offers more security than either packet filtering or circuit monitoring alone, but exacts a greater toll on network performance.

4.Application-level gateways

This kind of device, technically a proxy, and sometimes referred to as a proxy firewall, combines some of the attributes of packet filtering firewalls with those of circuit-level gateways. They filter packets not only according to the service for which they are intended -- as specified by the destination port -- but also by certain other characteristics, such as the HTTP request string.

5.Next-gen firewalls

This looser category is the most recent -- and least-well delineated -- of the types of firewalls. A typical next-gen product combines packet inspection with state ful inspection, but also includes some variety of deep packet inspection.

courtesy: http://searchsecurity.techtarget.com/feature/The-five-different-types-of-firewalls

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