Before we go into some of the complexities of networked software, it's important to understand how the Internet itself is designed. In many ways, the Internet functions the same as a large network that you might have at your home or office—the difference, of course, is that instead of workstations, the Internet has Internet-facing services. Most homes, for example, have many computers that are on the same Local Area Network (LAN), but only one IP address on the Internet. If someone were to scan this network from their Internet-connected device, they would only see Internet-facing services—not each family member's individual computer. This is important to note because across the Internet, only ports that are forwarded to a machine may be accessible on a given IP address. There are some exceptions to this rule that we will look at later.

When scanning a computer on your LAN, however, you will generally be able to see all the ports that are open. This is because...