A firewall is a system or group of systems that enforces an access control policy between two networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic.

Probably the most important thing to recognize about a firewall is that it implements an access control policy. If you don't have a good idea of what kind of access you want to allow or to deny, a firewall really won't help you.

It's also important to recognize that the firewall's configuration, because it is a mechanism for enforcing policy, imposes its policy on everything behind it. Administrators for firewalls managing the connectivity for a large number of hosts therefore have a heavy responsibility.

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

1. Service Critical Resource
2. Email
3. Disk I/O
4. Netnews Disk I/O
5. Web Host
6. OS Socket Performance
7. IP Routing Host OS Socket Performance
8. Web Cache
9. Host OS Socket Performance, Disk I/O

Each site is a little different from every other in terms of what attacks are likely to be used against it. Some recurring themes do arise, though.

A network gateway joins two networks together through a combination of hardware and software. A network firewall guards a computer network against unauthorized incoming or outgoing access. Network firewalls may be hardware devices or software programs.