DHCP Primer Page 3 — NAT, Ports, and Fooling the Outside World While your DHCP server is messing around with your addresses inside your network, it holds on to a single IP address outside your network. Whenever you send a packet of information out to the Internet, your DHCP server translates the client's internal IP address to your external IP address. To outside users, including your ISP, all of the traffic coming from and going in to your house originates from one IP address. Of course, with your fancy dynamic addressing, you can have as many computers as you desire humming away in your house, and your ISP will never know the difference. Once your network is DHCP-equipped, you can use the protocol to make otherwise complicated tasks -- such as hosting your own website in your home -- a breeze. Suppose your dream is to become the next Raymond Carver, or at least the next big Internet writer hotshot. Of course you want to get your work out there for would-be readers to click through, so you decide to turn your old Pentium 166 machine into a simple Web server. But you're concerned about your readers not being able to find your awesome site safely stashed on your private network, and rightly so. If you've been paying close attention, you'll remember that DHCP will keep your Web server hidden away from eager eyes. Users on the outside of your private home network will only see one IP address (your DHCP server's) at your location and thus will not be able to access your Web server. Luckily, that problem is easily fixed. Many of the DHCP-enabled software and hardware packages we'll be discussing on the next few pages allow you to set up ports (which are like "detours" for Internet traffic) on your server to allow certain kinds of traffic to reach specific applications on your network. Defining a port will link any incoming data to the desired application. For example, you can direct all incoming traffic intended for your Web server (such as HTTP requests) to port 80 on your network. The server will recognize the traffic as a request for the Web server and will direct the traffic to the appropriate internal IP address. Finally, your complex and broodingly intellectual masterpiece has found an audience! Port numbers, such as 80 for HTTP, 21 for FTP, and 23 for telnet, are standardized across the Internet. If you're interested, the Information Sciences Institute has a massive list of all available port numbers. Now you've got your homegrown website alive and kicking, but you're a jet-set kind of power writer, fond of taking your laptop with you to coffee house readings and lunches with David Foster Wallace. You may ask, "How will DHCP handle the mobility factor?" In a world without dynamic addressing, you would have to configure your laptop to use a unique IP address for each of your various networks (home, work, library, etc.) every time you log in -- how confusing! With DHCP running on your network, all you need to do is plug in your network cable and boot up your machine. Your PC will request a temporary IP address from the DHCP server and your connection is established. This way, you can take your laptop with you and wherever you go, there's a connection waiting for you. Now that all this talk about how easy it is to share your speedy Internet connection has you salivating all over your ergonomic keyboard, let's dig right in. We'll start with the easy solution (hardware) and then move on to the more challenging path (software), both of which arrive at the same results. To next page of DHCP To last page of DHCP Back to the HTML Home Page