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Computer networks make our lives easier. We have more information at the tips of our fingers now than at any other time in history. We can buy things from around the world with the click of a virtual button.

It's brilliant. It's also dangerous.

Networks aren't just convenient, they're complicated systems designed to facilitate the transfer of billions of bits of data every second. If history has taught us anything, it's that if someone creates a complicated system, someone else will want to figure out how it works and how they can make it work in ways different from the way the designer intended. When the specific system is a network, we call the people who use the system with unauthorized intent "hackers". Some people take offense to the term "hacker", claiming that the true identification should be "cracker" for someone who has malicious intent, and a "hacker" is someone who is just curious and interested in finding out how the system works. It's semantics. Hacker or cracker, the title isn't necessarily important, a person who is intent upon breaking into a computer system is the bad guy. Period.

So how do the good guys stop the bad guys? Layers.

The two major categories for network security are host-based and network-based. Host-based security focuses on each individual computer on the network (for the sake of this article, a computer is a server, laptop, or desktop, not a network appliance or device such as a router or switch). Network-based security focuses on the kinds of data that crosses into and out of the network. Both have their benefits and problems. Using both creates a more secure environment.

So let's take a look at some of the methods used to secure a computer network.

Firewall

A firewall is, essentially, a device designed to permit and deny specific types of traffic, based upon rules written by a security administrator. A firewall is designed to have traffic enter one of its network interfaces and exit another of its network interfaces (an interface is where the network cable gets plugged in). In order for the firewall to allow traffic from one interface to another, it must have rules that allow that traffic to go from one interface to another.

For example, if one of the interfaces is called "external" and the other (there are often more than two interfaces used on a firewall, but for the sake of simplicity, only two are needed to illustrate how it works) interface is called "internal". The external interface, because

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