Dynamic IP Addressing Versus Fixed IP Assignment

If your Internet connection works using a dynamic IP (Internet Protocol) address, it means that every time your computer starts up, it is assigned an IP address from the pool of available addresses. The same thing is true when your IP is released and renewed using Ipconfig. In other words, you don't have a fixed IP address on the Internet. You can think of dynamic IP addressing as similar to time-sharing at a holiday resort: You can pack more travelers in because nobody owns anything specific, but you don't know for sure exactly who will be at a specific location at any given time.

Dynamic IP addressing is the most common way ISPs such as your cable or DSL provider use to assign you an IP address on the Internet. The IP address that the ISP dynamically provides you is used by your computer to identify itself on the Internet. It's also used by an access point/router to identify your whole network to the Internet. (The router also assigns IP addresses within your network, but that's a different story.)

Note

Generally, if you are setting a static IP address for your network, you'll also need to provide an address for a DNS server. DNS is further explained later in this chapter.

But there are some things you can't do with a dynamic IP. For example, in order to host a website from your computer (as opposed to a site that an ISP hosts), you need to have a fixed (or static) IP address. You need a "location," meaning Internet IP address, at which your website can always be found.

Note

If your ISP uses dynamic, rather than fixed, IP addressing, then the IP settings will be empty (there is nothing to read).

So for this reason, and for other reasons known only to themselves, some ISPs provide you with one or more fixed IPs, rather than dynamically generated IPs.

If you have a fixed IP, you will need to enter it in your access point/router, or it will not be capable of accessing the Internet.

TIP

For an explanation about how to open your Internet Protocol (TCP/IP) Properties dialog, see Chapter 13.

You can ask your ISP about the IP settings you need. Alternatively, if you are running Windows XP and are connected to the Internet, you can read the fixed IP settings from the Internet Protocol (TCP/IP) Properties dialog, as shown in Figure 15.1.

Figure 15.1. You can read your fixed IP and DNS settings off your computer connected to the Internet using the Internet Protocol (TCP/IP) Properties dialog.

Static on the OutsideDynamic Inside!

It's really important to understand that even if your ISP uses a fixed IP for your connection to the Internet, the computers on your LAN (the local area network in your home or office) most likely use dynamic IP addresses. These dynamic IP addresses are assigned by the access point.

So after the access point router has been loaded with your fixed IP settings, you need to set each computer on your network to accept dynamic IP addressing. You can think of the access point/router as acting on behalf of all the computers on your network. It connects to the network using whatever settingsfixed or dynamic IPsand then, on its own initiative, it assigns IP addresses to your computers within your network.

In some cases, you might want to assign a static IP address to a computer on your private network. This computer then has a fixed IP address, even though the other devices on your network have addresses assigned dynamically. One reason you could assign a fixed IP address to a computer is to prevent it from accessing the Internet. For example, suppose that you want your kids to be able to share files on your network, but not to surf the Internet. For more information, see "IP Filtering" later in this chapter.

Now that you've got the IP (and DNS) settings required by your network, you can open the administrative application for the access point/router. Figure 15.2 shows configuring the Linksys Wireless Broadband Router by selecting Static IP as the Internet Connection type, and then entering the information you gathered in the previous section. Click Apply to enter these new settings in the access point/router.

Figure 15.2. The static IP settings are entered in the access point/router, using the device's administration panel.