“Any sufficiently advanced technology is indistinguishable from magic,” states science-fiction titan Arthur C. Clarke’s third law. The Internet barely existed when the author of 2001: A Space Odyssey proposed his law, but it certainly can seem supernatural. Still, the Internet’s magic is accompanied by a heap of trouble in the form of spam, viruses, spyware, zombie PCs, phishers, pharmers, and the other nasty side-effects of digital life. You can avoid trouble more easily if you know a bit about how the Internet works. Here is my executive guide to the essential acronyms of the Internet.

IP address: If your system is connected to a local network or to the Internet, it almost certainly relies on an Internet Protocol address to move data packets. Three IP address ranges are reserved for private networks, such as the one in your home or office: 169.254.xxx.xxx, 10.10.xxx.xxx, and 192.168.xxx.xxx (where ‘xxx’ is any number from 0 to 255). If your computer uses a public address (any address outside the private address ranges noted above), it is susceptible to attacks from other PCs on the network. This is why a Windows XP system that’s not patched against such direct attacks as the Blaster worm will crash within a few minutes of being connected through a public IP address. Windows assigns 169.254 addresses when it can’t see a DHCP server (see below). To find your IP address in Windows XP, right-click My Network Places and choose Properties. Select your LAN connection in the right pane, and your IP address will be listed under Details in the left pane (click the downward-pointing chevron to view the Details info, if necessary).

DHCP and NAT: The Dynamic Host Control Protocol allows one computer or other network device (such as a router) to hand out private IP addresses to other PCs. If your computer’s address lies in one of the private ranges noted above, a DHCP server probably assigned it. Similarly, Network Address Translation permits a device or computer (usually the same one that runs the DHCP server) to translate the public addresses on incoming packets into the private addresses that your local computers use. This enables you to browse the Web, send and receive e-mail, and connect to other Internet servers while staying invisible to attacking PCs. Even if your system is vulnerable to the Blaster worm, for example, NAT prevents the vermin from getting through.

DNS: The Web’s domain-name system lets you connect to PC World’s Web site using the domain name ‘www.pcworld.com’, instead of having to enter the site’s IP address. The end of the site name (the ‘.com’ in the case of ‘www.pcworld.com’) is called the top-level domain. The second-level domain (’.pcworld’) appears just to the left of the top-level domain. The ‘www’ is a third-level domain, which can be almost anything–a fact phishers exploit to deceive victims.

URL: The Uniform Resource Locator lets your browser and other programs on your PC connect to a remote server via the Internet. URLs contain the domain name (or alternatively, the IP address) of the remote server and often include the name of a file or subdirectory on the server. They are usually preceded by the Internet protocol (’http://’ for Web sites, for example); see Figure 1 A well-constructed malicious URL can do a lot of damage, especially if it takes advantage of a flaw in a program on your PC. To be safe when clicking URLs embedded in a Web site, an e-mail message, or elsewhere, be paranoid: Don’t do so until you know the link is safe. If the top- and second-level domains in the URL look phishy (if, for example, the second- and third-level domains have been swapped), don’t take the bait.