A firewall is a device that controls incoming and outgoing network traffic based on a set of rules. A bastion host is a special computer designed to withstand attacks by running only a single application like a proxy server and limiting other services. There are three main types of firewalls: packet filters, stateful filters, and application layer firewalls. Popular firewall brands include phion netfence, Astaro Security Linux, ActionTEC, and Arkoon FAST360.

1. How Corporate Firewalls Works
About Firewall
About Bastion host
Firewall's Types
Major Firewall

2. About Firewall
A firewall is a device
or set of devices
configured to permit,
deny, encrypt, or
proxy all computer
traffic between
different security
domains based upon
a set of rules and
other criteria.

3. About Bastion host
A bastion host is a special purpose computer
on a network specifically designed and
configured to withstand attack.
The computer hosts a single application.(for
example a proxy server)
And all other services are removed or limited
to reduce the threat to the computer.

4. Firewall's Types
packet filters type
"stateful" filters type
application layer type

5. Major Firewall
phion netfence
Astaro Security Linux
ActionTEC
Arkoon FAST360

6. References
http://ja.wikipedia.org/wiki/%E3%83%95%E3%82%A
http://en.wikipedia.org/wiki/Firewall
http://en.wikipedia.org/wiki/Bastion_host
http://e-words.jp/w/E38395E382A1E382A4E382A2